Process for producing unitary graphene materials

ABSTRACT

A process for producing a unitary graphene material, comprising: (a) preparing a graphene oxide (GO) gel having GO molecules dissolved in a fluid medium wherein the GO molecules contain higher than 20% by weight of oxygen; (b) dispensing and depositing a layer of GO gel onto a surface of a substrate to form a layer of deposited GO gel thereon, wherein the dispensing and depositing procedure includes shear-induced thinning; (c) removing the fluid medium from the deposited GO gel to form a GO layer having an inter-plane spacing d 002  of 0.4 nm to 1.2 nm as determined by X-ray diffraction; and (d) heat treating the GO layer to form the unitary graphene material at a heat treatment temperature higher than 100° C. to an extent that d 002  is decreased to a value of 0.3354 nm to 0.4 nm and the oxygen content is decreased to less than 5% by weight.

The present invention claims the benefits of the following co-pendingpatent applications: A. Zhamu, et al., “Graphene Oxide Gel BondedGraphene Composite Films and Processes for Producing Same,” U.S. patentapplication Ser. No. 13/385,813 (Mar. 8, 2012). Aruna Zhamu, MingchaoWang, Wei Xiong, and Bor Z. Jang, “Unitary Graphene Layer or GrapheneSingle Crystal,” U.S. patent application Ser. No. 13/694,356 (Nov. 26,2012).

Aruna Zhamu, Mingchao Wang, Wei Xiong, and Bor Z. Jang, “UnitaryGraphene Matrix Composites Containing Carbon or Graphite Fillers,” U.S.patent application Ser. No. 13/694,468 (Dec. 5, 2012).

Aruna Zhamu, Yi-jun Lin, Mingchao Wang, Wei Xiong, and Bor Z. Jang,“Unitary Graphene Material-Based Integrated Finned Heat Sink,” U.S.patent application Ser. No. 13/694,791 (Jan. 7, 2013).

FIELD OF THE INVENTION

The present invention relates generally to the field of graphiticmaterials for heat dissipation applications and, more particularly, to aprocess for producing a unitary graphene material. This unitary graphenematerial exhibits a combination of exceptionally high thermalconductivity, high electrical conductivity, high mechanical strength,good surface scratch resistance, and good hardness.

BACKGROUND OF THE INVENTION

Advanced thermal management materials are becoming critical for today'smicroelectronic, photonic, and photovoltaic systems. For instance, asnew and more powerful chip designs and light-emitting diode (LED)systems are introduced, they consume more power and generate more heat.This has made thermal management a crucial issue in today's highperformance systems. Systems ranging from active electronically scannedradar arrays, web servers, large battery packs for personal consumerelectronics, wide-screen displays, and solid-state lighting devices allrequire high thermal conductivity materials that can dissipate heat moreefficiently. On the other hand, the devices are designed and fabricatedto become increasingly smaller, thinner, lighter, and tighter. Thisfurther increases the difficulty of thermal dissipation. Actually,thermal management challenges are now widely recognized as the keybarriers to industry's ability to provide continued improvements indevice and system performance.

One group of materials potentially suitable for heat dissipationapplications is the graphitic carbon or graphite. Carbon is known tohave five unique crystalline structures, including diamond, fullerene(0-D nano graphitic material), carbon nano-tube or carbon nano-fiber(1-D nano graphitic material), graphene (2-D nano graphitic material),and graphite (3-D graphitic material). The carbon nano-tube (CNT) refersto a tubular structure grown with a single wall or multi-wall. Carbonnano-tubes (CNTs) and carbon nano-fibers (CNFs) have a diameter on theorder of a few nanometers to a few hundred nanometers. Theirlongitudinal, hollow structures impart unique mechanical, electrical andchemical properties to the material. The CNT or CNF is a one-dimensionalnano carbon or 1-D nano graphite material.

Bulk natural flake graphite is a 3-D graphitic material with eachparticle being composed of multiple grains (a grain being a graphitesingle crystal or crystallite) with grain boundaries (amorphous ordefect zones) demarcating neighboring graphite single crystals. Eachgrain is composed of multiple graphene planes that are oriented parallelto one another. A graphene plane in a graphite crystallite is composedof carbon atoms occupying a two-dimensional, hexagonal lattice. In agiven grain or single crystal, the graphene planes are stacked andbonded via van der Waal forces in the crystallographic c-direction(perpendicular to the graphene plane or basal plane). Although all thegraphene planes in one grain are parallel to one another, typically thegraphene planes in one grain and the graphene planes in an adjacentgrain are different in orientation. In other words, the orientations ofthe various grains in a graphite particle typically differ from onegrain to another.

A graphite single crystal (crystallite) per se is anisotropic with aproperty measured along a direction in the basal plane (crystallographica- or b-axis direction) being dramatically different than if measuredalong the crystallographic c-axis direction (thickness direction). Forinstance, the thermal conductivity of a graphite single crystal can beup to approximately 1,920 W/mK (theoretical) or 1,800 W/mK(experimental) in the basal plane (crystallographic a- and b-axisdirections), but that along the crystallographic c-axis direction isless than 10 W/mK (typically less than 5 W/mK). Further, the multiplegrains or crystallites in a graphite particle are typically all orientedalong different directions. Consequently, a natural graphite particlecomposed of multiple grains of different orientations exhibits anaverage property between these two extremes (i.e. between5 W/mK and1,800 W/mK.

It would be highly desirable in many applications to produce a bulkgraphite particle (containing single or multiple grains) havingsufficiently large dimensions and having all graphene planes beingessentially parallel to one another along one desired direction. Forinstance, it is highly desirable to have one large-size graphite entity(e.g. a fully integrated or unitary layer of multiple graphene planes)having the c-axis directions of all the graphene planes beingsubstantially parallel to one another and having a sufficiently largelength and/or width for a particular application (e.g. >5 cm² for use asa heat-spreading sheet on a CPU of a smart phone) and a sufficientthickness (e.g. >10 μm and more preferably >20 μm) to impart rigidity toa thin film for easy handling. It would be further desirable if such a“giant graphite particle” has only one grain or few grains (thus, no orlittle grain boundaries) and has few or no defects therein to impede theflow of electrons and phonons. Thus far, it has not been possible toproduce this type of large-size unitary graphene entity from existingnatural or synthetic graphite particles.

The constituent graphene planes of a graphite crystallite can beexfoliated and extracted or isolated from a graphite crystallite toobtain individual graphene sheets of carbon atoms provided theinter-planar van der Waals forces can be overcome. An isolated,individual graphene sheet of carbon atoms is commonly referred to assingle-layer graphene. A stack of multiple graphene planes bondedthrough van der Waals forces in the thickness direction with aninter-graphene plane spacing of 0.3354 nm is commonly referred to as amulti-layer graphene. A multi-layer graphene platelet has up to 300layers of graphene planes (<100 nm in thickness), but more typically upto 30 graphene planes (<10 nm in thickness), even more typically up to20 graphene planes (<7 nm in thickness), and most typically up to 10graphene planes (commonly referred to as few-layer graphene inscientific community). Single-layer graphene and multi-layer graphenesheets are collectively called “nano graphene platelets” (NGPs).Graphene sheets/platelets or NGPs are a new class of carbon nanomaterial (a 2-D nano carbon) that is distinct from the 0-D fullerene,the 1-D CNT, and the 3-D graphite.

Our research group pioneered the development of graphene materials andrelated production processes as early as 2002: (1) B. Z. Jang and W. C.Huang, “Nano-scaled Graphene Plates,” U.S. Pat. No. 7,071,258 (Jul. 4,2006), application submitted on Oct. 21, 2002; (2) B. Z. Jang, et al.“Process for Producing Nano-scaled Graphene Plates,” U.S. patentapplication Ser. No. 10/858,814 (Jun. 3, 2004); and (3) B. Z. Jang, A.Zhamu, and J. Guo, “Process for Producing Nano-scaled Platelets andNanocomposites,” U.S. patent application Ser. No. 11/509,424 (Aug. 25,2006).

NGPs are typically obtained by intercalating natural graphite particleswith a strong acid and/or oxidizing agent to obtain a graphiteintercalation compound (GIC) or graphite oxide (GO), as illustrated inFIG. 1( a) (process flow chart) and FIG. 1( b) (schematic drawing). Thepresence of chemical species or functional groups in the interstitialspaces between graphene planes serves to increase the inter-graphenespacing (d₀₀₂, as determined by X-ray diffraction), therebysignificantly reducing the van der Waals forces that otherwise holdgraphene planes together along the c-axis direction. The GIC or GO ismost often produced by immersing natural graphite powder (20 in FIGS. 1(a) and 100 in FIG. 1( b)) in a mixture of sulfuric acid, nitric acid (anoxidizing agent), and another oxidizing agent (e.g. potassiumpermanganate or sodium perchlorate). The resulting GIC (22 or 102) isactually some type of graphite oxide (GO) particles. This GIC is thenrepeatedly washed and rinsed in water to remove excess acids, resultingin a graphite oxide suspension or dispersion, which contains discreteand visually discernible graphite oxide particles dispersed in water.There are two processing routes to follow after this rinsing step:

Route 1 involves removing water from the suspension to obtain“expandable graphite,” which is essentially a mass of dried GIC or driedgraphite oxide particles. Upon exposure of expandable graphite to atemperature in the range of typically 800-1,050° C. for approximately 30seconds to 2 minutes, the GIC undergoes a rapid expansion by a factor of30-300 to form “graphite worms” (24 or 104), which are each a collectionof exfoliated, but largely un-separated graphite flakes that remaininterconnected. A SEM image of graphite worms is presented in FIG. 2(a).

In Route 1A, these graphite worms (exfoliated graphite or “networks ofinterconnected/non-separated graphite flakes”) can be re-compressed toobtain flexible graphite sheets or foils (26 or 106) that typically havea thickness in the range of 0.1 mm (100 μm)-0.5 mm (500 μm).Alternatively, one may choose to use a low-intensity air mill orshearing machine to simply break up the graphite worms for the purposeof producing the so-called “expanded graphite flakes” (108) whichcontain mostly graphite flakes or platelets thicker than 100 nm (hence,not a nano material by definition).

Exfoliated graphite worms, expanded graphite flakes, and therecompressed mass of graphite worms (commonly referred to as flexiblegraphite sheet or flexible graphite foil) are all 3-D graphiticmaterials that are fundamentally different and patently distinct fromeither the 1-D nano carbon material (CNT or CNF) or the 2-D nano carbonmaterial (graphene sheets or platelets, NGPs). Flexible graphite (FG)foils can be used as a heat spreader material, but exhibiting a maximumin-plane thermal conductivity of typically less than 500 W/mK (moretypically <300 W/mK) and in-plane electrical conductivity no greaterthan 1,500 S/cm. These low conductivity values are a direct result ofthe many defects, wrinkled or folded graphite flakes, interruptions orgaps between graphite flakes, and non-parallel flakes (e.g. SEM image inFIG. 2( b)). Many flakes are inclined with respect to one another at avery large angle (e.g. mis-orientation of 20-40 degrees).

In Route 1B, the exfoliated graphite is subjected to high-intensitymechanical shearing (e.g. using an ultrasonicator, high-shear mixer,high-intensity air jet mill, or high-energy ball mill) to form separatedsingle-layer and multi-layer graphene sheets (collectively called NGPs,33 or 112), as disclosed in our U.S. application Ser. No. 10/858,814.Single-layer graphene can be as thin as 0.34 nm, while multi-layergraphene can have a thickness up to 100 nm, but more typically less than20 nm.

Route 2 entails ultrasonicating the graphite oxide suspension for thepurpose of separating/isolating individual graphene oxide sheets fromgraphite oxide particles. This is based on the notion that theinter-graphene plane separation has been increased from 0.3354 nm innatural graphite to 0.6-1.1 nm in highly oxidized graphite oxide,significantly weakening the van der Waals forces that hold neighboringplanes together. Ultrasonic power can be sufficient to further separategraphene plane sheets to form separated, isolated, or discrete grapheneoxide (GO) sheets. These graphene oxide sheets can then be chemically orthermally reduced to obtain “reduced graphene oxides” (RGO) typicallyhaving an oxygen content of 0.001%40% by weight, more typically 0.01%-5%by weight, most typically and highly desirably less than 2% by weight.

For the purpose of defining the claims of the instant application, NGPsinclude discrete sheets/platelets of single-layer and multi-layergraphene, graphene oxide, or reduced graphene oxide with an oxygencontent of 0-10% by weight, more typically 0-5% by weight, andpreferably 0-2% by weight. Pristine graphene has essentially 0% oxygen.Graphene oxide (including RGO) can have 0.001%-46% by weight of oxygen.

The GO molecules in graphene oxide gel, to be described in detail later,typically contains 20-50% by weight oxygen (more typically 30-47%)immediately after removal of the liquid from the GO gel, but prior to asubsequent heat treatment. The GO gel refers to a homogeneous solutionof highly hydrophilic aromatic molecules (graphene oxide moleculesbearing oxygen-containing groups, such as —OH, —COOH, and >O, onmolecular planes or at the edges) that are dissolved (not justdispersed) in a liquid (e.g. acidic water). The GO gel per se does notcontain visibly discernible or discrete graphene or GO particles in theform of solid sheets or platelets. These GO molecules and the dispersingliquid medium have comparable indices of refraction, making theresulting gel optically transparent or translucent (if the proportion ofGO molecules are bot excessively high), or showing lightly brown color.In contrast, the simple mixture of original graphite particles ordiscrete NGP sheets/platelets with acids and/or water appears opticallydark and totally opaque (even with only <0.1% solid particles suspendedin the liquid medium). These particles or NGP platelets are simplydispersed (not dissolved) in the fluid medium.

These GO molecules in a GO gel are highly reactive and may be consideredas “living giant molecules”. By contrast, the prior art solidsheets/platelets of graphene, GO, and RGO are essentially “dead”species. The GO gel can be formed into a shape with a proper shearing orcompression stress (e.g. via casting or molding), dried (with liquidcomponents partially or totally removed), and heat-treated under certainconditions to obtain a unitary graphene material, which is typically asingle crystal, a poly-crystal with incomplete or poorly delineatedgrain boundaries, or a poly-crystal with very large grain sizes (veryfew grains). The heat treatment serves to chemically link these activeor live GO molecules to form a 2-D or 3-D network of chemically bondedgraphene molecules of essentially infinite molecular weights, and todrastically reduce the oxygen content of GO down to below 10% by weight,more typically <5%, further more typically <2%, and most typically <<1%.Only a trace amount of oxygen (practically 0%) can survive if the heattreatment temperature is sufficiently high and heat treatment timesufficiently long. This new and unique material called “unitary graphenematerial” will be further described in detail later.

Although the GO gel per se does not contain visibly discernible/discretegraphene sheets/platelets or NGPs (including “dead” GOsheets/platelets), one can intentionally add discrete graphenesheets/platelets, expanded graphite flakes, and other type of solidfiller in the GO gel to form a mixture gel. This mixture gel may bedried and subjected to the same heat treatment to convert the live GOmolecules into a unitary graphene material, also enabling these activemolecules to chemically bond to the filler particles. This grapheneoxide gel-derived graphene material, reinforced with a filler phase(e.g. discrete NGPs, CNTs and carbon fibers), constitutes the presentlyinvented unitary graphene matrix composite as a readily mass-processiblematerial.

It may be noted that flexible graphite foils (obtained by compressing orroll-pressing exfoliated graphite worms) for electronic device thermalmanagement applications (e.g. as a heat sink material) have thefollowing major deficiencies:

-   -   (1) As indicated earlier, flexible graphite (FG) foils exhibit a        relatively low thermal conductivity, typically <500 W/mK and        more typically <300 W/mK. By impregnating the exfoliated        graphite with a resin, the resulting composite exhibits an even        lower thermal conductivity (typically <<200 W/mK, more typically        <100 W/mK).    -   (2) Flexible graphite foils, without a resin impregnated therein        or coated thereon, are of low strength, low rigidity, and poor        structural integrity. The high tendency for flexible graphite        foils to get torn apart makes them difficult to handle in the        process of making a heat sink. As a matter of fact, the flexible        graphite sheets (typically 50-200 μm thick) are so “flexible”        that they are not sufficiently rigid to make a fin component        material for a finned heat sink.    -   (3) Another very subtle, largely ignored or overlooked, but        critically important feature of FG foils is their high tendency        to get flaky with graphite flakes easily coming off from FG        sheet surfaces and emitting out to other parts of a        microelectronic device. These highly electrically conducting        flakes (typically 1-200 μm in lateral dimensions and >100 nm in        thickness) can cause internal shorting and failure of electronic        devices.    -   (4) Both resin-free flexible graphite and resin-impregnated FG        (with resin impregnating step occurring before or after        roll-pressing) are not conducive to mass production of finned        heat sink structures. It is virtually impossible to use mass        production processes (such as extrusion, stamping, forging, and        die casting that are commonly used for making aluminum heat        sinks, or injection molding for making conductive        filler-reinforced plastic-based heat sinks) to make FG-based        heat sinks without some kind of subsequent bonding or assembling        operations. One has to manually attach individual fin members to        a core or base member. For instance, one may produce bonded fin        heat sink assemblies in which each fin in the assembly is        individually bonded into a heat sink base. A major shortcoming        of such heat sinks is their high cost. This cost is related        directly to the labor required to individually arrange each fin        on some sort of support or substrate (a base or core) and high        production cycle time. Further, bonding between a fin and a base        is not always reliable and the long-term reliability of flexible        graphite-based finned heat sinks is highly questionable.

Similarly, solid NGPs (including discrete sheets/platelets of pristinegraphene, GO, and GRO), when packed into a film, membrane, or papersheet (34 or 114) of non-woven aggregates, typically do not exhibit ahigh thermal conductivity unless these sheets/platelets are closelypacked and the film/membrane/paper is ultra-thin (e.g. <1 μm, which ismechanically weak). This is reported in our earlier U.S. patentapplication Ser. No. 11/784,606 (Apr. 9, 2007). However, ultra-thin filmor paper sheets (<10 μm) are difficult to produce in mass quantities,and difficult to handle when one tries to incorporate these thin filmsas a heat sink material. In general, a paper-like structure or mat madefrom platelets of graphene, GO, or RGO (e.g. those paper sheets preparedby vacuum-assisted filtration process) exhibit many defects, wrinkled orfolded graphene sheets, interruptions or gaps between platelets, andnon-parallel platelets (e.g. SEM image in FIG. 3( b)), leading torelatively poor thermal conductivity, low electric conductivity, and lowstructural strength. These papers or aggregates of discrete NGP, GO orRGO platelets alone (without a resin binder) also have a tendency to getflaky, emitting conductive particles into air.

Our earlier application (U.S. application Ser. No. 11/784,606) alsodisclosed a mat, film, or paper of NGPs infiltrated with a metal, glass,ceramic, resin, and CVD carbon matrix material (graphenesheets/platelets being the filler or reinforcement phase, not the matrixphase in this earlier application). Haddon, et al. (US Pub. No.2010/0140792, Jun. 10, 2010) also reported NGP thin-film paper(aggregates of isolated platelets) and NGP-reinforced polymer matrixcomposites for thermal management applications. The processes used byHaddon et al to produce NGPs are identical to those disclosed muchearlier by us (Jang, et al. U.S. patent application Ser. No. 10/858,814(Jun. 3, 2004)). The NGP-reinforced polymer matrix composites, as anintended thermal interface material, have very low thermal conductivity,typically <<2 W/mK. The NGP films of Haddon, et al are essentiallynon-woven aggregates of discrete graphene platelets, identical to thoseof our earlier invention (U.S. application Ser. No. 11/784,606). Again,these aggregates have a great tendency to have graphite particlesflaking and separated from the film surface, creating internal shortingproblem for the electronic device containing these aggregates. They alsoexhibit low thermal conductivity unless made into thin films (10 nm-300nm, as reported by Haddon, et al) which are very difficult to handle ina real device manufacturing environment. Balandin, et al (US Pub. No.2010/0085713, Apr. 8, 2010) also disclosed a graphene layer produced byCVD deposition or diamond conversion for heat spreader application. Morerecently, Kim, et al (N. P. Kim and J. P. Huang, “Graphene NanoplateletMetal Matrix,” US Pub. No. 2011/0108978, May 10, 2011) reported metalmatrix infiltrated NGPs. However, the metal matrix is too heavy and theresulting metal matrix composite does not exhibit a high thermalconductivity. More significantly, all these prior art materials andrelated processes are not amenable to mass production ofheat-dissipation structures, such as finned heat sinks,cost-effectively. In fact, there has been no known report on using thesematerials for finned heat sink applications.

Another prior art material for thermal management applications is thepyrolitic graphite film. The lower portion of FIG. 1( a) illustrates atypical process for producing prior art pyrolitic graphitic films from apolymer. The process begins with carbonizing a polymer film 46 (e.g.polyimide) at a carbonization temperature of 400-1,000° C. under atypical pressure of 10-15 Kg/cm² for 2-10 hours to obtain a carbonizedmaterial 48, which is followed by a graphitization treatment at2,500-3,200° C. under an ultrahigh pressure of 100-300 Kg/cm² for 1-24hours to form a graphitic film 50. It is technically utmost challengingto maintain such an ultrahigh pressure at such an ultrahigh temperature.This is a difficult, slow, tedious, energy-intensive, and extremelyexpensive process. Furthermore, carbonization of certain polymers (e.g.polyacrylonitrile) involves the emission of toxic species.

A second type of pyrolytic graphite is produced by high temperaturedecomposition of hydrocarbon gases in vacuum followed by deposition ofthe carbon atoms to a substrate surface. This vapor phase condensationof cracked hydrocarbons is essentially a chemical vapor deposition (CVD)process. In particular, highly oriented pyrolitic graphite (HOPG) is thematerial produced by the application of uniaxial pressure on depositedpyrocarbon or pyrolytic graphite at very high temperatures (typically3,000-3,300° C.). This entails a thermo-mechanical treatment of combinedand concurrent mechanical compression and ultra-high temperature for anextended period of time in a protective atmosphere; a very expensive,energy-intensive, and technically challenging process. The processrequires ultra-high temperature equipment (with high vacuum, highpressure, or high compression provision) that is not only very expensiveto make but also very expensive and difficult to maintain. Even withsuch extreme processing conditions, the resulting PG (including HOPG)still possesses many defects, grain boundaries, and mis-orientations(neighboring graphene planes not parallel to each other), resulting inless-than-satisfactory in-plane properties. Typically, the best preparedHOPG sheet or block remains far from being a graphite single crystal;instead, it typically still contains many grains or crystals and a vastamount of grain boundaries and defects. All PG film production processesdo not allow for impregnation of a resin matrix. PG or HOPG films, beingweak, non-rigid, and not easily processable suffer from the sameshortcomings as flexible graphite intended for use to construct finnedheat sinks.

Similarly, the most recently reported graphene thin film (<2 nm)prepared by catalytic CVD of hydrocarbon gas (e.g. C₂H₄) on Ni or Cusurface is not a single-grain crystal, but a poly-crystalline structurewith many grain boundaries and defects. With Ni or Cu being thecatalyst, carbon atoms obtained via decomposition of hydrocarbon gasmolecules at 800-1,000° C. are deposited onto Ni or Cu foil surface toform a sheet of single-layer or few-layer graphene that ispoly-crystalline. The grains are typically much smaller than 100 μm insize and, more typically, smaller than 10 μm in size. These graphenethin films, being optically transparent and electrically conducting, areintended for applications such as the touch screen (to replaceindium-tin oxide or ITO glass) or semiconductor (to replace silicon,Si). However, these ultra-thin polycrystalline graphene films are notsufficiently thermally conducting (too many grains or too much grainboundaries, and all grains being oriented in different directions) andnot sufficiently thick for use as a heat sink or heat spreader material.Furthermore, the Ni- or Cu-catalyzed CVD process does not lend itself tothe deposition of more than 5-10 graphene planes (typically <2-4 nm)beyond which the underlying Ni or Cu catalyst can no longer provide anycatalytic effect. There has been no experimental evidence to indicatethat CVD graphene layer thicker than 5 or 10 nm is possible. Further,CVD is not conducive to the fabrication of a heat sink that is typicallycomplex in shape.

Thus, it is an object of the present invention to provide a process forproducing graphene oxide (GO) gel-derived unitary graphene material(monolithic graphene entity) or its composite version (containing, forinstance, a carbon/graphite filler phase dispersed in or bonded by aunitary graphene matrix material derived from a GO gel), which exhibitsa thermal conductivity comparable to or greater than the thermalconductivities of the PG (including HOPG), CVD graphene film, and/orflexible graphite (including resin-impregnated FG).

This thermally and electrically conductive graphene monolith or graphenematrix composite can be used to produce finned heat sinks costeffectively in large quantities, using commonly used, less complex, andeasier-to-control processes with readily available, inexpensiveequipment.

It is another object of the present invention to provide a process forproducing GO-derived unitary graphene entity and graphene matrixcomposite that exhibit a combination of exceptional thermalconductivity, electrical conductivity, mechanical strength, surfacehardness, and scratch resistance unmatched by any material of comparablethickness range.

It is a specific object of the present invention to provide a highlyconductive unitary graphene material or graphene matrix composite thatmeets the following technical requirements (a) a thermal conductivitygreater than 600 W/mK (preferably greater than 1,000 W/mK, and furtherpreferably greater than 1,700 W/mK); (b) an electrical conductivitygreater than 2,000 S/cm (preferably >3,000 S/cm, more preferably >5,000S/cm, even more desirably >10,000 S/cm, and most preferably >15,000S/cm); (c) Rockwell surface hardness value >60 (preferably >80); and/or(d) a tensile or flexural strength greater than 80 MPa (preferably >100MPa, more preferably >150 MPa, and most preferably >200 MPa). No priorart material meets this set of technical requirements.

This new class of materials (i.e., a GO gel-derived unitary graphenemonolithic and the unitary graphene matrix composite) has the followingcharacteristics (separately or in combination) that distinguishthemselves from PG, HOPG, CVD graphene film, flexible graphite sheets,flexible graphite composite, paper/film/membrane of discretegraphene/GO/RGO sheets/platelets, and conventional graphene/GO/RGOplatelet-reinforced resin matrix composite, metal matrix composite, andcarbon matrix composites:

(1) This unitary graphene material is an integrated graphene entity thatis either a graphene single crystal (single grain only) or apoly-crystal (multiple grains but typically having incomplete grainboundaries or having exceptionally large grains). Typically andpreferably, with some compression or shearing stresses exerted on the GOduring shaping and subsequent heat treating, the unitary graphenematerial has all the graphene planes in all the grains being essentiallyoriented parallel to one another (i.e., the crystallographic c-axis ofall grains pointing in an identical direction).

(2) The unitary graphene matrix is an integrated graphene entity that isnot an aggregate or stack of multiple discrete graphite flakes ordiscrete platelets of graphene/GO/RGO, and does not contain anydiscernible or discrete flake/platelet derived from the original GO gel.

(3) This integrated graphene matrix is not made by bonding discreteflakes/platelets together with a binder or adhesive. Instead, GOmolecules in the GO gel are chemically active and live species capableof chemically merging with one another mainly in an edge-to-edge manner(forming 2-D giant graphene molecules), but possibly also with adjacentGO molecules below or above (forming 3-D network of graphene chains).Through joining or forming of covalent bonds with one another, the GOmolecules are adhered into an integrated graphene entity (the unitarygraphene material), without using any externally added linker or bindermolecules or polymers. In the presence of discretecarbon/graphite/graphene filler particles (e.g. carbon black particles,CNTs, and NGPs), the GO molecules are also capable of acting as a binderor adhesive that chemically bonds these carbon/graphite filler particlestogether to form a strong composite.

(4) This unitary or monolithic graphene matrix (a single crystal orpoly-crystal with essentially all graphene planes having an identicalcrystallographic c-axis) is derived from a GO gel, which is in turnobtained from heavy oxidation of natural graphite or artificial graphiteparticles originally having multiple graphite crystallites. Prior tobeing chemically oxidized to become GO gel, these starting or originalgraphite crystallites have an initial length (L_(a) in thecrystallographic a-axis direction), initial width (L_(b) in the b-axisdirection), and thickness (L_(c) in the c-axis direction). The resultingunitary graphene entity typically has a length or width significantlygreater than the L_(a) and L_(b) of the original graphite crystallites.

(5) It may be noted that there has been numerous reports on “graphenecomposites.” However, these “graphene composites” make use of discretepristine graphene sheets, graphene oxide platelets, or reduced grapheneoxide platelets as the reinforcement phase, which is dispersed in amatrix material selected from a resin (to form a resin matrixcomposite), a metal (metal matrix composite), a carbon (carbon matrixcomposite), a glass (glass matrix composite), or a ceramic (ceramicmatrix composite). In these prior art “graphene composites,” graphenesheets/platelets are the discrete and dispersed phase, not the matrixphase (i.e., not the continuous phase that bonds and protects thedispersed phase). These discrete graphene sheets/platelets are thedispersed phase bonded and protected by a matrix material, such as aresin, metal, carbon (CVD carbon, amorphous carbon, or polymericcarbon), glass, or ceramic. In stark contrast or completely oppositely,in the presently invented unitary graphene matrix composite, graphene isthe matrix material that serves to bond, adhere, and protect thedispersed filler phase, such as CNT and carbon black (CB) particles. CNTor CB particles are dispersed in and protected by the unitary graphenematrix. The graphene matrix is a continuous, unified, or integratedmaterial phase.

The present invention also provides a cost-effective method or processfor producing a GO gel-derived unitary graphene entity and the graphenematrix composite on a continuous or roll-to-roll basis.

Another object of the present invention is to provide a cost-effectiveprocess for producing a GO-derived graphene monolith or a graphenematrix composite that exhibits a combination of exceptional thermalconductivity, electrical conductivity, mechanical strength, surfacehardness, and scratch resistance.

In particular, the present invention provides a fast, scalable processcapable of mass-producing unitary or monolithic graphene or graphenematrix composites from a GO gel. Advantageously and surprisingly,conventional mass production techniques, such as die casting, injectionmolding, compression molding, resin-transfer molding, and extrusion, canbe adapted for cost-effectively producing these materials.

This process involves significantly lower heat treatment temperatures ascompared with the processes for producing pyrolytic graphite (includingHOPG) from either carbonized polymers (e.g. polyimide) or the CVDgraphite. The presently invented process is simpler (hence, morereliable), less energy-intensive, and highly scalable.

SUMMARY OF THE INVENTION

The present invention provides a process for producing a unitarygraphene material, the process comprising: (a) preparing a grapheneoxide gel having graphene oxide molecules dispersed in a fluid mediumwherein the graphene oxide molecules contain an oxygen content higherthan 20% by weight; (b) dispensing and depositing a layer of grapheneoxide gel onto a surface of a supporting substrate to form a depositedgraphene oxide gel thereon, wherein the dispensing and depositingprocedure includes shear-induced thinning of the graphene oxide gel; (c)partially or completely removing the fluid medium from the depositedgraphene oxide gel layer to form a graphene oxide layer having aninter-plane spacing d₀₀₂ of 0.4 nm to 1.2 nm as determined by X-raydiffraction and an oxygen content no less than 20% by weight; and (d)heat treating the graphene oxide layer to form said unitary graphenematerial at a heat treatment temperature higher than 100° C. to anextent that an inter-plane spacing d₀₀₂ is decreased to a value of from0.3354 nm to 0.4 nm and the oxygen content is decreased to less than 5%by weight.

Shear-induced thinning in step (b) means the GO gel is subjected to ashear stress during processing and a viscosity of the GO gel is reducedduring and/or after the application of such a shear stress. As anexample, the shear stress can be encountered in a situation as simple asa “doctor's blade” that guides the spreading of GO gel over a glasssurface during a manual casting process. As another example, aneffective shear stress is created in an automated roll-to-roll coatingprocess in which a “knife-on-roll” configuration dispenses GO over amoving solid substrate, such as a plastic film. The relative motionbetween this moving film and the coating knife creates such a shearstress.

This shear-induced thinning is a critically important step in theproduction of the presently invented unitary graphene material due tothe surprising observation that shear-induced thinning enables the GOmolecules to align themselves along a particular direction (e.g.X-direction) or two particular directions (e.g. X- and Y-directions) toproduce preferred orientations. Further surprisingly, these preferredorientations are preserved and often further enhanced during thesubsequent heat treatment to produce the unitary graphene material. Mostsurprisingly, such preferred orientations are essential to the eventualattainment of exceptionally high thermal conductivity, electricalconductivity, and mechanical strength of the resulting unitary graphenematerial along a desired direction. These great properties in thisdesired direction could not be obtained without such a shearstress-induced orientation control.

In one embodiment, the graphene oxide gel is obtained by immersingpowders or filaments of a graphitic material in an oxidizing liquidmedium (e.g. a mixture of sulfuric acid, nitric acid, and potassiumpermanganate) in a reaction vessel. The graphitic material may beselected from natural graphite, artificial graphite, meso-phase carbon,meso-phase pitch, meso-carbon micro-bead, soft carbon, hard carbon,coke, carbon fiber, carbon nano-fiber, carbon nano-tube, or acombination thereof. When the graphite powders or filaments are mixed inthe oxidizing liquid medium, the resulting slurry initially appearscompletely dark and opaque. The resulting mass is simply a heterogeneoussuspension of solid particles dispersed (not dissolved) in a liquidmedium. When the oxidation of graphite proceeds at a reactiontemperature for a sufficient length of time under a controlled pHcondition, the reacting mass can eventually turn optically translucent,transparent (if sufficiently dilute), or uniform brown color which alsolooks and behaves like a gel. This heavy oxidation-induced grapheneoxide gel is composed of graphene oxide molecules uniformly dissolved inthe liquid medium. We observe that even if the initial solid graphitepowder particles dispersed in water occupy a proportion as low as 0.1%by weight or lower, the suspension is heterogeneous and looks completelydark and opaque. In contrast, the GO gel is a homogeneous solution. Evenwhen the GO molecule content exceeds 1% by weight, the GO gel can appeartranslucent or transparent.

The graphene oxide molecules in the GO gel, prior to any subsequent heattreatment, have an oxygen content no less than 20% by weight (moretypically greater than 30% by weight, and most typically from 40-50% byweight) and their molecular weights are typically less than 43,000g/mole (often less than 4,000 g/mole, but typically greater than 200g/mole) while in a gel state. The graphene oxide gel is composed ofgraphene oxide molecules dissolved (not just dispersed) in an acidicmedium having a pH value of typically no higher than 5.

Subsequently, the GO gel is formed into a shape (e.g. cast film on asolid substrate or sheared mass in a mold cavity) with the liquidcomponent in the GO gel being partially or completely removed to obtainat least partially dried GO mass containing well-packed and well-alignedlive GO molecules.

In one embodiment, the graphene oxide molecules in step (a) contain anoxygen content higher than 30% by weight.

In another embodiment, step (c) includes forming a graphene oxide layerhaving an inter-plane spacing d₀₀₂ of 0.4 nm to 0.7 nm and an oxygencontent no less than 20% by weight; and step (d) includes heat-treatingthe graphene oxide layer to an extent that an inter-plane spacing d₀₀₂is decreased to a value of from 0.3354 nm to 0.36 nm and the oxygencontent is decreased to less than 2% by weight.

In still another embodiment, the graphene oxide gel has a viscositygreater than 2,000 cP (centipoise) when measured at 20° C. prior to theshear-induced thinning procedure, but the viscosity is reduced to below2,000 cP (or even below 1,000 cP) during or after shear-inducedthinning. In still another embodiment, the graphene oxide gel has aviscosity greater than 5,000 cP when measured at 20° C. prior toshear-induced thinning, but is reduced to below 5,000 cps (preferablyand typically below 2,000 cP or even below 1,000 cP) during or aftershear-induced thinning. Preferably, the graphene oxide gel has aviscosity from 500 cP to 500,000 cP when measured at 20° C. prior toshear-induced thinning.

Preferably, the graphene oxide gel has a viscosity less than 5,000 cP(preferably less than 2,000 cP and further preferably less than 1,000cP) when measured at 20° C. after shear-induced thinning. In general,the graphene oxide gel has a viscosity that decreases by at least 10times when a shear rate is increased to a finite extent at 20° C. Theshear-induced thinning may be conducted via any of a wide variety ofprocedures, including coating, casting, injection molding, compressionmolding, resin-transfer molding, extrusion, pultrusin, orfilament-winding.

The dried GO mass after deposition or molding is then subjected to aproperly programmed heat treatment that can be divided into fourdistinct temperature regimes. The presently invented unitary graphenematerial can be obtained by heat-treating the dried GO mass with atemperature program that covers at least the first regime, more commonlycovers the first two regimes, still more commonly the first threeregimes, and most commonly all the 4 regimes (the latter beingimplemented to achieve the highest conductivity):

-   Regime 1: 100° C.-500° C. (the thermal reduction regime); Oxygen    content reduced from typically 30-50% to 5-6%, resulting in a    reduction of inter-graphene spacing from approximately 0.6-1.0 nm to    approximately 0.4 nm and an increase in in-plane thermal    conductivity of a consolidated thin film from approximately 100 to    450 W/mK.-   Regime 2: 500° C.-1,250° C. (the chemical linking regime); Oxygen    content reduced to typically 0.7% (<<1%), resulting in a reduction    of inter-graphene spacing to approximately 0.345 nm, an increase in    in-plane thermal conductivity of a unitary graphene thin film to    1,400-1,500 W/mK, and/or in-plane electrical conductivity to    3,000-4,000 S/cm.-   Regime 3: 1,250° C.-2,000° C. (the ordering and re-graphitization    regime); Oxygen content reduced to typically 0.01%, resulting in a    reduction of inter-graphene spacing to approximately 0.337 nm    (degree of graphitization from 1% to approximately 80%) and improved    degree of ordering, an increase in in-plane thermal conductivity of    a unitary graphene thin film to >1,680 W/mK, and/or in-plane    electrical conductivity to 5,000-7,000 S/cm.-   Regime 4: 2,000° C.-3,000° C. (the re-crystallization and perfection    regime); Oxygen content reduced to typically from near 0%-0.001%,    resulting in a reduction of inter-graphene spacing to approximately    0.3354 nm (degree of graphitization from 80% to nearly 100%) and    perfection of crystal structure and orientation, an increase in    in-plane thermal conductivity of a unitary graphene thin film    to >1,800 W/mK, and in-plane electrical conductivity to    15,000-25,000 S/cm.

The degree of graphitization, g, was calculated from the X-raydiffraction pattern using Mering's Eq, d₀₀₂=0.3354 g+0.344 (1−g), whered₀₀₂ is the interlayer spacing of graphite or graphene crystal in nm.This equation is valid only when d₀₀₂ is no greater than 0.3440 nm. Theunitary graphene material having a d₀₀₂ higher than 0.3440 nm reflectsthe presence of oxygen-containing functional groups (such as —OH, >O,and —COOH on graphene molecular plane surfaces) that act as a spacer toincrease the inter-graphene spacing.

Another structural index that can be used to characterize the degree ofordering of the presently invented unitary graphene material or relatedgraphite crystals is the “mosaic spread” value, which is expressed bythe full width at half maximum of the (002) or (004) reflection in aX-ray diffraction intensity curve. This degree of ordering characterizesthe graphite or graphene crystal size (or grain size), amounts of grainboundaries and other defects, and the degree of preferred grainorientation. A nearly perfect single crystal of graphite ischaracterized by having a mosaic spread value of 0.2-0.4. Most of ourunitary graphene materials have a mosaic spread value in this range of0.2-0.4 (with a heat treatment temperature no less than 2,000° C.).However, some values are in the range of 0.4-0.7 if the highest heattreatment temperature (TTT) is between 1,250 and 2,000° C., and in therange of 0.7-1.0 if the TTT is between 500 and 1,250° C.

It is very important to state that the presently invented process allowsus to prepare unitary graphene materials from an ultra-thin thickness(e.g. 10 nm to 100 nm, and up to 1 μm thin films), through intermediatethickness (1 μm-500 μm thick films), and high thickness (>0.5 mm for abulk material). This is a significant advantage that no other graphiticor graphene material processes could provide. For instance, flexiblegraphite sheets produced from exfoliated worms could not be thicker than50 μm (mostly >100 μm). Polyimide-derived pyrolytic graphite film istypically limited to 20 μm-50 μm due to the nature of availableprecursor polymer dimensions and carbonization conditions. Catalytic CVDgraphene cannot go over 5-10 nm in thickness. Non-catalytic CVD can bethicker, but the graphene sheets tend to be randomly oriented.

Thus, one embodiment of the invention is a process that can produceunitary graphene material layer having a thickness greater than 100 nm.Another embodiment is a process for producing a unitary graphenematerial having a thickness greater than 100 nm but less than 10 μm.Another embodiment is a process for producing a unitary graphenematerial thicker than 10 μm. Still another embodiment is a process forproducing a unitary graphene material having a thickness greater than100 μm, further preferably >500 μm. However, with a thickness greaterthan 500 μm or 0.5 mm, the degree of graphene plane orientation is notnearly as high as in a unitary graphene layer thinner than 0.5 mm.

A preferred embodiment of the present invention is a unitary graphenematerial or a unitary graphene material being reinforced with a filleror reinforcement phase, wherein the unitary graphene material exhibits amosaic spread value less than 1.0. Preferably, the unitary graphenematerial exhibits a degree of graphitization no less than 40% and/or amosaic spread value less than 0.7. Further preferably, the unitarygraphene material exhibits a degree of graphitization no less than 80%and/or a mosaic spread value no greater than 0.4. Most preferably, thedegree of graphitization is at least 99% and/or the mosaic spread valueis from 0.2 to 0.4, representing a near perfect graphene single crystalhaving all graphene planes essentially perfectly parallel to oneanother.

The presently invented unitary graphene material can further contain adiscrete filler or reinforcement phase dispersed in the unitary graphenematerial to form a unitary graphene matrix composite structure. Thefiller or reinforcement phase may contain a particle, filament,nano-tube, nano-wire, or nano-rod of a metal, ceramic, glass, polymer,carbon, graphite, or a combination thereof. Particularly desired filleror reinforcement phase is selected from a carbon or graphite fiber,carbon or graphite nano-fiber, carbon nano-tube, carbon nano-rod,meso-phase carbon particle, meso-carbon micro-bead, expanded graphiteflake with a thickness greater than 100 nm, single-layer graphene sheet,multi-layer graphene platelet with a thickness less than 100 nm,exfoliated graphite or graphite worm, coke particle, needle coke, carbonblack or acetylene black particle, activated carbon particle, or acombination thereof. The carbon, graphite, or graphene filler phaseoccupies a weight fraction of 0.01% to 99% (preferably from 10% to 70%)based on the total composite structure weight.

Preferably and typically, the carbon or graphite filler is chemicallybonded by the unitary graphene matrix. It is most surprising that thisunitary graphene matrix, prepared through the route of a GO gel, iscapable of chemically bonding to a filler phase and that the constituentGO molecules in a GO gel mass are capable of chemically bonding andmerging with one another to form an integrated 2-D or 3-D network ofaromatic chains or giant graphene molecules of essentially infinitemolecular weight, much like a 3-D network of cross-linked polymerchains. Chemical analyses, including various spectroscopy studies, havedemonstrated that these chemically bonded graphene molecules contain acombination of sp² and sp³ electronic configurations.

It may be noted that the unitary graphene matrix material, when preparedalone without the presence of the carbon or graphite filler phase, canbe made into a unitary graphene layer or graphene single crystal. Thisunitary graphene layer or graphene single crystal would contain closelypacked and bonded parallel graphene planes having an inter-grapheneplane spacing of 0.3354 to 0.40 nm (mostly between 0.3354 and 0.337 nm)and an oxygen content up to 10% by weight (mostly <<1%). This unitarygraphene layer or graphene single crystal can be obtained fromheat-treating a graphene oxide gel at a temperature higher than 100° C.(up to 500, 1,250, 2,000, or 3,000° C., depending upon the desiredproperties), wherein an average mis-orientation angle between twographene planes is less than 10 degrees, preferably and typically lessthan 5 degrees. The graphene single crystal, prepared alone without thepresence of a filler, refers to the single-grain or single-domaingraphene or poly-crystalline structure (but having incomplete grainboundaries) in which most of the graphene planes in all grain(s) areessentially parallel to one another. This unitary graphene or graphenemonolith contains therein no discrete graphite flake or grapheneplatelet derived from the graphene oxide gel. All graphene oxidemolecules have been chemically merged, linked, and integrated into onesingle integral unit, hence the name “unitary graphene” entity.

The unitary graphene matrix or the graphene matrix composite typicallyand preferably has a physical density of at least 1.7 g/cm³ or aporosity level lower than 10%, and more typically and preferably has aphysical density of at least 1.8 g/cm³ or a porosity level lower than5%. The process enables us to produce unitary graphene material, withouta filler, to reach a physical density most typically in the range of2.0-2.25 g/cm³, approaching the theoretical density of a perfectgraphite single crystal. Yet, no conventional graphite single crystalcan be readily produced to have a dimension larger than a few microns(μm). We can produce this giant graphene particle wider or longer thantens of centimeters that are practically a single crystal. This is mostastonishing.

In an embodiment, the graphene oxide gel is obtained from a graphiticmaterial having a maximum original graphite grain size and the unitarygraphene material is a single crystal or a poly-crystal graphenestructure having a grain size larger than even the maximum originalgrain size. This maximum original grain size is the length or width of agraphene plane or a graphite crystallite in a graphite particle prior tobeing oxidized. The heat treatment involves extensive merging andlinking of highly reactive GO molecules to form huge graphene planes andhuge graphene domains (or grains) that are typically orders of magnitudegreater than the original grain sizes.

In the unitary graphene matrix composite prepared in the presence of afiller phase, the chemically bonded graphene planes also can be parallelto one another (e.g. along a fiber axis direction). In the unitarygraphene matrix composite, the unitary graphene matrix typicallycontains no complete grain boundary therein and contains no discrete ordiscernible graphene platelet derived from the original graphene oxidegel. Preferably and typically, the carbon or graphite filler ischemically bonded by the unitary matrix material in the composite (e.g.via covalent bonds).

The production of the graphene matrix composite typically begins withpreparation of a mass of GO gel, which is then mixed with particles ofthe carbon/graphite filler phase to form a slurry mass. The slurry isformed into a desired shape (a finned heat sink or a component)supported by a die casting tool or mold cavity, preferably with a shearstress to facilitate orientation or alignment of aromatic GO molecules.Alternatively, the carbon/graphite filler phase is first formed into aporous preform (e.g. mat, paper, or fabric) of a desired heat sinkshape, which is then impregnated with the GO gel. In either route, theliquid component of this GO gel is then partially or totally removedand, concurrently or sequentially, this GO material is subjected to aheat treatment. This heat treatment, also herein referred to as achemical linking and re-graphitization treatment, thermally converts theGO molecules to an integrated graphene entity by chemically mergingindividual graphene oxide molecules primarily sideway in an edge-to-edgemanner to form significantly larger graphene planes, but sometimes alsochemically linking with the GO molecules below or above this grapheneplane to form a 3-D molecular network. This 3-D molecular network can bebroken and re-organized if the final heat treatment occurs at asufficiently high temperature for an extended length of time.

Further alternatively, the carbon or graphite filler may be made into aform of fiber yarns or fiber bundles impregnated with the graphene oxidegel with shear-induced thinning, and the composite is made by formingthe fiber yarns or bundles into a desired shape prior to heat treating.The desired shape can mean part of or an entire heat sink structure. Itis highly surprising for us to observe that graphene oxide gel has anoutstanding adhesive power that can bond the filler phase (e.g. carbonfibers or nano-tubes) together to form a composite of exceptionalstructural integrity.

The graphene oxide gel-derived unitary or monolithic graphene entity orthe corresponding graphene matrix composite has a unique combination ofoutstanding thermal conductivity, electrical conductivity, mechanicalstrength, scratch resistance, and elimination of the possibility ofhaving surface graphite flakes or particles to “flake off” (actually,there is no discrete flake/platelet to be peeled therefrom).

Further, the graphene oxide (GO) gel-derived unitary graphene matrixmaterial or graphene matrix composite has the following novel, unique,and unprecedented characteristics (separately or in combination):

-   -   (1) The unitary graphene matrix material itself is an integrated        graphene object that is either a graphene single crystal or a        poly-crystal having multiple grains (but with incomplete or        poorly delineated grain boundaries, or huge grain sizes, having        negligible amount of grain boundaries that would otherwise        impede flow of electrons and phonons). When made into a        thin-film form (e.g. <200 μM thick) or formed under the        influence of a shear stress (to induce viscosity thinning), the        unitary graphene matrix is composed of multiple graphene planes        most of which are essentially oriented parallel to one another.    -   (2) In contrast to the paper-like sheets of expanded graphite        flakes or graphene platelets (e.g. those prepared by a        paper-making process), this integrated graphene entity (the        unitary graphene material) is not an aggregate or stack of        multiple discrete graphite flakes or discrete platelets of        graphene, GO, or RGO. This is a single graphene entity or        monolith, not a simple aggregate of multiple graphite flakes        (such as in FG foil) or graphene sheets/platelets (such as prior        art graphene paper/membrane/film). This unitary graphene entity        does not contain discrete graphite flakes or discrete graphene        platelets dispersed therein that are derived from the GO gel.        The GO molecules do not revert back to individual or discrete        graphene platelets or graphite flakes. Through chemical        inter-linking of GO molecules, re-graphitization, and        re-crystallization, the GO molecules and the original graphene        planes of hexagonal carbon atoms (constituting original graphite        particles) have completely lost their original individual        identity and have been united into one single entity (unitary        body or monolith).    -   (3) The integrated graphene entity is not made by gluing or        bonding discrete flakes/platelets together with a binder,        linker, or adhesive. Instead, GO molecules in the GO gel are        merged, mainly edge-to-edge through joining or forming of        covalent bonds with one another, into an integrated graphene        entity, without using any externally added linker or binder        molecules or polymers.    -   (4) This unitary or monolithic graphene entity is derived from a        GO gel, which is in turn obtained from natural graphite or        artificial graphite particles originally having multiple        graphite crystallites. Prior to being chemically oxidized, these        starting graphite crystallites have an initial length (L_(a) in        the crystallographic a-axis direction), initial width (L_(b) in        the b-axis direction), and thickness (L_(c) in the c-axis        direction). The resulting unitary graphene entity typically has        a length or width significantly greater than the L_(a) and L_(b)        of the original crystallites. Even the individual grains in a        poly-crystalline unitary graphene entity have a length or width        significantly greater than the L_(a) and L_(b) of the original        crystallites. They can be as large as the length or width of the        unitary graphene entity itself, not just 2 or 3 times higher        than the initial L_(a) and L_(b) of the original crystallites.        The unitary graphene layer or graphene single crystal typically        has a length or width no less than 10 μm, more typically no less        than 100 μm, and even more typically no less than 1 cm. They        often are extended to cover the entire width of the original GO        gel layer deposited on a substrate surface, which can be >100 cm        as desired.

The unitary graphene matrix material can have a thickness as low as 10nm and can be as low as 2 nm, but the ultra-thin film thinner than 2 nmhas the tendency to get fragmented when heated to a high temperature.The unitary graphene material preferably has a thickness >100 nm, morepreferably >1 μm, even more preferably >10 μm. The unitary graphenematrix material alone or graphene matrix composite preferably has athickness less than 200 μm for a heat spreader application, but it canbe much thicker. The thickness range of 20-100 μm is particularly usefulfor mobile device thermal management applications.

A specific embodiment of the present invention is a process forproducing a unitary graphene matrix composite, comprising: (a) preparinga graphene oxide gel having graphene oxide molecules dissolved in afluid medium to form a homogeneous solution; (b) mixing a carbon orgraphite filler phase in the graphene oxide gel to form a slurry; (c)dispensing the slurry into a cavity of a molding tool or forming theslurry into a desired shape under the influence of a shear stress tocreate shear-induced thinning and molecular orientation; (d) partiallyor completely removing the fluid medium from the slurry to form acomposite precursor; and (e) heat-treating the composite precursor toform the unitary graphene composite at a temperature higher than 100° C.(can be higher than 500° C., 1,250° C., 2000° C., or even 3,000° C. ifso desired).

Another specific embodiment is a process for producing a unitarygraphene matrix composite, comprising: (a) preparing a graphene oxidegel having graphene oxide molecules dissolved in a fluid medium to forma homogeneous solution; (b) combining a carbon or graphite filler phasewith the graphene oxide gel to form an impregnated filler shape underthe influence of a shear stress that induces viscosity thinning; (c)partially or completely removing the fluid medium from the impregnatedfiller shape to form a composite precursor; and (d) heat-treating thecomposite precursor to form the unitary graphene composite at atemperature higher than 100° C. (can be higher than 500° C., 1,250° C.,2000° C., or even 3,000° C. if so desired).

The unitary graphene matrix composite shows a surprisingly high Rockwellhardness value, typically greater than 80 and often greater than 100.This is unprecedented since prior art flexible graphite foil, pyrolyticgraphite, or bulk graphite does not show such a high hardness.

The unitary graphene matrix composite of the present invention canexhibit an electrical conductivity greater than 1,500 S/cm, a thermalconductivity greater than 600 W/mK, a physical density greater than 1.8g/cm3, and/or a tensile strength greater than 80 MPa. With a higherre-graphitization temperature, the graphene monolithic can have anelectrical conductivity greater than 3,000 S/cm, a thermal conductivitygreater than 1,000 W/mK, a physical density greater than 1.9 g/cm3,and/or a tensile strength greater than 100 MPa. It can even exhibit anelectrical conductivity greater than 5,000 S/cm, a thermal conductivitygreater than 1,500 W/mK, a physical density greater than 2.0 g/cm³,and/or a tensile strength greater than 150 MPa.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 (a) A flow chart illustrating various prior art processes ofproducing exfoliated graphite products (flexible graphite foils andflexible graphite composites) and pyrolytic graphite (bottom portion),along with a process for producing graphene oxide gel 21, oriented GOlayer 35, and unitary graphene material 37; (b) Schematic drawingillustrating the processes for conventional producing paper, mat, film,and membrane of simply aggregated graphite or NGP flakes/platelets. Allprocesses begin with intercalation and/or oxidation treatment ofgraphitic materials (e.g. natural graphite particles).

FIG. 2 (a) A SEM image of a graphite worm sample after thermalexfoliation of graphite intercalation compounds (GICs) or graphite oxidepowders; (b) An SEM image of a cross-section of a flexible graphitefoil, showing many graphite flakes with orientations not parallel to theflexible graphite foil surface and also showing many defects, kinked orfolded flakes.

FIG. 3 (a) A SEM image of a GO-derived graphene monolithic whereinmultiple graphene planes (having a length/width of 30 nm-2 μm) ingraphite particles, have been oxidized, exfoliated, re-oriented, andseamlessly merged into continuous-length graphene sheets or layers thatcan run for hundreds of centimeters wide or long (only a 120 μm or 0.12mm width of a 25-cm wide unitary graphene material being shown in thisSEM image); (b) A SEM image of a cross-section of a conventionalgraphene paper/film prepared from discrete graphene sheets/plateletsusing a paper-making process (e.g. vacuum-assisted filtration). Theimage shows many discrete graphene sheets being folded or interrupted(not integrated), with orientations not parallel to the film/papersurface and having many defects or imperfections; (c) Schematic drawingand an attendant SEM image to illustrate the formation process of aunitary graphene entity or graphene single crystal that is composed ofmultiple graphene planes that are parallel to one another and arechemically bonded in the thickness-direction or crystallographic c-axisdirection; (d) Schematic of the prior art graphene poly-crystal obtainedby CVD of hydrocarbon on a catalytic surface (e.g. Cu or Ni); (e)Schematic of a graphene single crystal of the present invention; (f)Schematic of another graphene single crystal of the present invention (a“poly-crystal” with incomplete grain boundaries); (g) One plausiblechemical linking mechanism (only 2 GO molecules are shown as an example;a large number of GO molecules can be chemically linked together to forma unitary graphene layer).

FIG. 4 (a) Thermal conductivity values of the GO gel-derived singleunitary graphene layer (▴), GO platelet paper (▪), and FG foil (♦)plotted as a function of the final heat treatment temperature forgraphitization or re-graphitization; (b) Thermal conductivity values ofthe GO gel-derived unitary graphene layer (▪) and the polyimide-derivedpyrolytic graphite (PG) heat-treated for one hour (x) and for 3 hours(▴), all plotted as a function of the final graphitization orre-graphitization temperature; (c) Electric conductivity values of theGO gel-derived unitary graphene layer (♦), GO platelet paper (▪), and FGfoil (x) plotted as a function of the final graphitization orre-graphitization temperature; and (d) thermal conductivity values ofunitary graphene layer only, unitary graphene matrix/CNT composite, GOpaper (prepared from GO platelets not reaching a GO gel state), andGO/CNT paper or membrane. Note: symbol designations varied from (a) to(d).

FIG. 5 X-ray diffraction curves of (a) a GO film (dried GO gel), (b) GOfilm thermally reduced at 150° C. (partially re-duced), (c) highlyreduced and re-graphitized GO film (a unitary graphene layer), (d)highly re-graphitized and re-crystallized GO single crystal (a moreadvanced unitary graphene material) showing a high-intensity (004) peak,and (e) a polyimide-derived HOPG with a HTT as high as 3,000° C.

FIG. 6 (a) Inter-graphene plane spacing measured by X-ray diffraction;(b) the oxygen content in the GO gel-derived unitary graphene layer; (c)correlation between inter-graphene spacing and the oxygen content; and(d) thermal conductivity of GO gel-derived unitary graphene layer andflexible graphite (FG) foil, all plotted as a function of the final heattreatment temperature.

FIG. 7 (a) Thermal conductivity values of the GO gel-derived unitarygraphene layer alone (▪), unitary graphene matrix-expanded graphitereinforcement composite (♦, experimental values), expanded graphite matalone (exfoliated graphite worms broken up into separated graphiteflakes and clustered into a thin mat) and FG foil alone (▴,re-compressed worms without worm break-up and flake separation as apoint of reference) plotted as a function of the final graphitization orre-graphitization temperature, along with theoretically predicted values(x, unitary graphene matrix-expanded graphite composite) based on arule-of-mixture law (final graphitization time=1 hour for allspecimens); (b) Thermal conductivity values of the GO gel-derivedunitary graphene layer alone (▪), unitary graphene matrix-expandedgraphite composite (♦), and polyimide-derived pyrolytic graphite (PG)plotted as a function of the final graphitization or re-graphitizationtemperature for one hour, along with those of PG graphitized for 3hours.

FIG. 8 (a) Tensile strength of unitary graphene matrix material from GOgel, paper of GO platelets (not from GO gel state), and flexiblegraphite foil over a range of heat treatment temperatures; (b) Tensilestrength and (c) Rockwell hardness values of unitary graphene matrix/CNTreinforcement composites, unitary graphene matrix/expanded graphitereinforcement composites, and unitary graphene matrix/carbon blackreinforcement composites plotted as a function of the filler weightpercentage, and (d) Rockwell hardness of unitary graphene matrixmaterial only and its CNT-reinforced version plotted as a function ofthe heat treatment temperature.

FIG. 9 Viscosity values of graphene gel plotted as a function ofviscometer spindle speed (proportional to a shear rate): (a)linear-linear scale, (b) log-linear scale, and (c) log-log scale.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a process for producing a unitarygraphene material, the process comprising:

(a) preparing a graphene oxide gel having graphene oxide moleculesdispersed in a fluid medium wherein the graphene oxide molecules containan oxygen content higher than 20% by weight (typically higher than 30%and more typically between 30% and 46% by weight);(b) dispensing and depositing a layer of graphene oxide gel onto asurface of a supporting substrate to form a deposited graphene oxide gelthereon, wherein the dispensing and depositing procedure includesshear-induced thinning of the graphene oxide gel (resulting in grapheneoxide molecules well-packed and well-aligned in desired direction(s),conducive to merging and integration of GO molecules during a subsequentheat treatment);(c) partially or completely removing the fluid medium from the depositedgraphene oxide gel layer to form a graphene oxide layer having aninter-plane spacing d₀₀₂ of 0.4 nm to 1.2 nm as determined by X-raydiffraction and an oxygen content no less than 20% by weight; and(d) heat treating the graphene oxide layer to form said unitary graphenematerial at a heat treatment temperature higher than 100° C. to anextent that an inter-plane spacing d₀₀₂ is decreased to a value of from0.3354 nm to 0.4 nm and the oxygen content is decreased to less than 5%by weight.

In a more preferred embodiment, step (c) includes forming a grapheneoxide layer having an inter-plane spacing d₀₀₂ of 0.4 nm to 0.7 nm andan oxygen content no less than 20% by weight; and step (d) includesheat-treating the graphene oxide layer to an extent that an inter-planespacing d₀₀₂ is decreased to a value of from 0.3354 nm to 0.36 nm andthe oxygen content is decreased to less than 2% by weight (mostpreferably between 0.001% to 0.01% by weight).

The unitary graphene material is obtained from heat-treating a grapheneoxide gel at a temperature higher than 100° C. (preferably higher than500° C., more preferably higher than 1,250° C., further preferablyhigher than 2,000° C., and advantageously higher than 2,500° C. if aperfect or nearly perfect graphene single crystal is desired) andcontains chemically bonded graphene molecules. These planar aromaticmolecules or graphene planes (hexagonal structured carbon atoms) areparallel to one another. The lateral dimensions (length or width) ofthese planes are huge, typically several times or even orders ofmagnitude larger than the maximum or “crystallite dimension (or maximumconstituent graphene plane dimension) of the starting graphiteparticles. The presently invented unitary graphene material is a “giantgraphene crystal” or “giant graphene particle” having all constituentgraphene planes being essentially parallel to one another. This is aunique and new class of material that has not been previouslydiscovered, developed, or suggested to possibly exist.

The graphene oxide gel is a very unique and novel class of material thatsurprisingly has great cohesion power (self-bonding, self-polymerizing,and self-crosslinking capability) and adhesive power (capable ofchemically bonding to a wide variety of solid surfaces). Thesecharacteristics have not been taught or hinted in the prior art. The GOgel is obtained by immersing powders or filaments of a startinggraphitic material in an oxidizing liquid medium (e.g. a mixture ofsulfuric acid, nitric acid, and potassium permanganate) in a reactionvessel. The starting graphitic material may be selected from naturalgraphite, artificial graphite, meso-phase carbon, meso-phase pitch,meso-carbon micro-bead, soft carbon, hard carbon, coke, carbon fiber,carbon nano-fiber, carbon nano-tube, or a combination thereof.

When the starting graphite powders or filaments are mixed in theoxidizing liquid medium, the resulting slurry (heterogeneous suspension)initially appears completely dark and opaque. When the oxidation ofgraphite proceeds at a reaction temperature for a sufficient length oftime under a controlled pH condition, the reacting mass can eventuallybecome a homogeneous solution having no discernible or visuallyidentifiable dispersed solid particle (as opposed to the initiallyheterogeneous suspension that contain identifiable solid particles). Thesolution can be optically translucent or transparent or brown-colored,which also looks and behaves like a polymer gel. This heavyoxidation-induced graphene oxide gel is composed of graphene oxidemolecules dissolved in the liquid medium. The graphene oxide molecules,prior to any subsequent heat treatment, have an oxygen content no lessthan 20% by weight (typically from 40-50% by weight) and their molecularweights are typically less than 43,000 g/mole (often less than 4,000g/mole, but typically greater than 200 g/mole) while in a gel state. Thegraphene oxide gel is composed of graphene oxide molecules dissolved inan acidic medium having a pH value of typically no higher than 5.

The graphene oxide gel has a typical viscosity from 500 centipoise (cP)to 500,000 cP when measured at 20° C. prior to shear-induced thinning.The viscosity is more typically greater than 2,000 cP and less than300,000 cP when measured at 20° C. prior to the shear-induced thinningprocedure. Preferably, the viscosity of the GO gel as a precursor tounitary graphene material is in the range of 2,000-50,000 cP.Preferably, the GO gel is subjected to a shear stress field that theviscosity is reduced to below 2,000 cP (or even below 1,000 cP) duringor after shear-induced thinning. In an embodiment, the graphene oxidegel has a viscosity greater than 5,000 cP when measured at 20° C. priorto shear-induced thinning, but is reduced to below 5,000 cP (preferablyand typically below 2,000 cP or even below 1,000 cP) during or aftershear-induced thinning. The viscosity data measured at 20° C., shown inFIGS. 9( a), 9(b), and 9(c) as an example, clearly indicate that even anultra-high viscosity value (e.g., 300,000 cP) can be reduced down to1,000-2,000 cP with a sufficiently high shear rate. This is a reductionby more than 2 orders of magnitude, a highly unexpected observation. Thestraight line of the data when plotted in a log-log scale indicates ashear thinning fluid flow behavior.

In step (b), the GO gel is formed into a shape preferably under theinfluence of a shear stress. One example of such a shearing procedure iscasting or coating a thin film of GO gel (gel-like fluid) using acoating machine. This procedure is similar to a layer of varnish, paint,coating, or ink being coated onto a solid substrate. The roller,“doctor's blade”, or wiper creates a shear stress when the film isshaped, or when a relative motion is effected between theroller/blade/wiper and the supporting substrate. Quite unexpectedly andsignificantly, such a shearing action reduces the effective viscosity ofthe GO gel and enables the planar graphene oxide (GO) molecules to wellalign along, for instance, a shearing direction. Further surprisingly,such a molecular alignment state or preferred orientation is notdisrupted when the liquid components in the GO gel are subsequentlyremoved to form a well-packed GO mass that is at least partially dried.The dried GO mass has a high birefringence coefficient between anin-plane direction and the normal-to-plane direction. Another example ofsuch a procedure is injecting or die-casting a GO mass into a moldcavity or shaping die/tooling under the influence of a shearing stress.The liquid component of the sheared GO mass in a mold cavity is thenpartially or completely removed to obtain a partially or totally driedGO mass containing well-packed and well-aligned “live” GO molecules.

This dried GO mass is then subjected to a properly programmed heattreatment that can be divided into four distinct heat treatmenttemperature (HTT) regimes:

-   Regime 1 (100° C.-500° C.): In this temperature range (the thermal    reduction regime), the GO mass primarily undergoes thermally-induced    reduction reactions, leading to a reduction of oxygen content from    typically 30-50% (as dried) to 5-6%. This treatment results in a    reduction of inter-graphene spacing from approximately 0.6-1.0 nm    (as dried) to approximately 0.4 nm and an increase in in-plane    thermal conductivity from approximately 100 W/mK to 450 W/mK. Even    with such a low temperature range, some chemical linking occurs. The    GO molecules remain well-aligned, but the inter-GO spacing remains    relative large (0.4 nm or larger). Many O-containing functional    groups survive.-   Regime 2 (500° C.-1,250° C.): In this chemical linking regime,    extensive chemical combination, polymerization, and cross-linking    between adjacent GO molecules occur. The oxygen content is reduced    to typically 0.7% (<<1%), resulting in a reduction of inter-graphene    spacing to approximately 0.345 nm. This implies that some initial    graphitization has already begun at such a low temperature, in stark    contrast to conventional graphitizable materials (such as carbonized    polyimide film) that typically require a temperature as high as    2,500° C. to initiate graphitization. This is another distinct    feature of the presently invented unitary graphene material and its    production processes. These chemical linking reactions result in an    increase in in-plane thermal conductivity of a unitary graphene thin    film to 1,400-1,500 W/mK, and/or in-plane electrical conductivity to    3,000-4,000 S/cm.-   Regime 3 (1,250° C.-2,000° C.): In this ordering and    re-graphitization regime, extensive graphitization or graphene plane    merging occurs, leading to significantly improved degree of    structural ordering. As a result, the oxygen content is reduced to    typically 0.01% and the inter-graphene spacing to approximately    0.337 nm (achieving degree of graphitization from 1% to    approximately 80%, depending upon the actual HTT and length of    time). The improved degree of ordering is also reflected by an    increase in in-plane thermal conductivity to >1,680 W/mK, and/or    in-plane electrical conductivity to 5,000-7,000 S/cm.-   Regime 4 (2,000° C.-3,000° C. or higher): In this re-crystallization    and perfection regime, extensive movement and elimination of grain    boundaries and other defects occur, resulting in the formation of    perfect or nearly perfect single crystals, or poly-crystalline    graphene crystals with incomplete grain boundaries or huge grains    (these grains can be orders of magnitude larger than the original    grain sizes of the starting graphite particles for GO gel    production. The oxygen content is essentially eliminated, typically    0%-0.001%. The inter-graphene spacing is reduced to down to    approximately 0.3354 nm (degree of graphitization from 80% to nearly    100%), corresponding to that of a perfect graphite single crystal.    Quite interestingly, the graphene single crystal or poly-crystal has    all the graphene planes being closely packed and bonded and all    aligned along one direction, a perfect orientation. Such a perfectly    oriented structure has not been produced even with the HOPG being    subjected concurrently to an ultra-high temperature (3,400° C.)    under an ultra-high pressure (300 Kg/cm²). The unitary graphene    entity car achieve such a highest degree of perfection with a    significantly lower temperature and an ambient (or slightly higher    compression) pressure. The unitary graphene material thus obtained    exhibits an in-plane thermal conductivity up to slightly >1,800    W/mK, and in-plane electrical conductivity to 15,000-25,000 S/cm.    The presently invented unitary graphene material can be obtained by    heat-treating the dried GO mass with a temperature program that    covers at least the first regime (typically requiring 1-4 hours in    this temperature range if the temperature never exceeds 500° C.),    more commonly covers the first two regimes (1-2 hours preferred),    still more commonly the first three regimes (preferably 0.5-2.0    hours in Regime 3), and most commonly all the 4 regimes (Regime 4,    for 0.2 to 1 hour, may be implemented to achieve the highest    conductivity).

X-ray diffraction patterns were obtained with an X-ray diffractometerequipped with CuKcv radiation. The shift and broadening of diffractionpeaks were calibrated using a silicon powder standard. The degree ofgraphitization, g, was calculated from the X-ray pattern using theMering's Eq, d₀₀₂=0.3354 g+0.344 (1−g), where d₀₀₂ is the interlayerspacing of graphite or graphene crystal in nm. This equation is validonly when d₀₀₂ is equal or less than approximately 0.3440 nm. Theunitary graphene material or lightly oxidized graphite crystallinematerial having a d₀₀₂ higher than 0.3440 nm reflects the presence ofoxygen-containing functional groups (such as —OH, >0, and —COOH ongraphene molecular plane surfaces) that act as a spacer to increase theinter-graphene spacing.

Another structural index that can be used to characterize the degree ofordering of the presently invented unitary graphene material andconventional graphite crystals is the “mosaic spread,” which isexpressed by the full width at half maximum of a rocking curve (X-raydiffraction intensity) of the (002) or (004) reflection. This degree ofordering characterizes the graphite or graphene crystal size (or grainsize), amounts of grain boundaries and other defects, and the degree ofpreferred grain orientation. A nearly perfect single crystal of graphiteis characterized by having a mosaic spread value of 0.2-0.4. Most of ourunitary graphene materials have a mosaic spread value in this range of0.2-0.4 (with a heat treatment temperature no less than 2,000° C.).However, some values are in the range of 0.4-0.7 if the highest heattreatment temperature (TTT) is between 1,250 and 2,000° C., and in therange of 0.7-1.0 if the TTT is between 500 and 1,250° C.

The present invention provides a process for producing a unitarygraphene material or a unitary graphene matrix composite composed of aunitary graphene as a matrix material (the continuous phase) and CNT orcarbon fibers as a discrete filler phase. In one preferred embodiment,the unitary graphene matrix composite is composed of: (a) a unitarygraphene matrix containing closely packed and chemically bonded grapheneplanes (preferably having an inter-graphene plane spacing of 0.3354 to0.40 nm and, optionally, an oxygen content of 0.001% to 10% by weight),which unitary graphene matrix is obtained from heat-treating a grapheneoxide gel at a temperature higher than 100° C.; and (b) A filler orreinforcement phase (e.g. particles or filaments of carbon, graphite,metal, glass, ceramic, and/or polymer).

Preferably, the reinforcement phase contains a carbon or graphite fillerphase selected from a carbon or graphite fiber, carbon or graphitenano-fiber, carbon nano-tube, carbon nano-rod, meso-phase carbonparticle, meso-carbon micro-bead, exfoliated graphite flake with athickness greater than 100 nm, exfoliated graphite or graphite worm,coke particle, needle coke, carbon black or acetylene black particle,activated carbon particle, or a combination thereof. The reinforcementphase occupies a weight fraction of 0.01% to 99% (preferably 10% to 70%)based on the total composite weight. The carbon or graphite filler phaseis preferably in a particulate, filamentary, or rod-like form dispersedin the unitary graphene matrix. These discrete particles, filaments, andcylindrical shape fillers are the dispersed phase (reinforcement orfiller phase) and the GO-derived unitary graphene material is thecontinuous phase (matrix). Preferably and typically, most of thechemically bonded graphene planes in the unitary graphene matrix areparallel to one another. Typically, the carbon or graphite filler ischemically bonded by the unitary graphene matrix material. This chemicalbonding is more pronounced if the carbon/graphite filler is chemicallytreated (e.g. using a mixture of sulfuric acid and nitric acid) prior tobeing mixed with the GO gel.

The heat treatment temperature conditions for GO are such that theunitary graphene material or the unitary graphene matrix composite isrelatively pore-free having a physical density of at least 1.5 g/cm³ ora porosity level lower than 20%. Under more typical processingconditions, the unitary graphene or the unitary graphene matrixcomposite has a physical density of at least 1.7 g/cm³ or a porositylevel lower than 10%. In most cases, the unitary graphene or the unitarygraphene matrix composite has a physical density greater than 1.8 g/cm³or a porosity level less than 5%. The chemically bonded graphene planesin the unitary graphene or graphene matrix composite typically contain acombination of sp² and sp³ electronic configurations (particularly forthose unitary graphene materials prepared with the maximum treatmenttemperature lower than 2,000° C.).

In a preferred embodiment of the present invention, the process forproducing the unitary graphene matrix composite comprises: (a) preparinga graphene oxide gel having graphene oxide molecules dissolved in afluid medium to form a homogeneous solution, wherein the graphene oxidegel is optically transparent, translucent, or brown colored; (b) mixingthe carbon or graphite filler phase in the graphene oxide gel to form aslurry; (c) dispensing the slurry into a cavity of a molding tool orforming the slurry into a desired shape under the influence of a shearstress (to create shear-induced thinning and molecular orientation); (d)partially or completely removing the fluid medium from the slurry toform a composite precursor; and (e) heat-treating the compositeprecursor to form the unitary graphene composite at a temperature higherthan 100° C. (preferably >500° C., more preferably >1,250° C., oreven >2,000° C.). Although not required, higher temperatures may be usedif so desired.

In this process, steps (c) and (d) preferably include feeding a sheet ofa solid substrate material from a roller to a deposition zone,dispensing the slurry or suspension onto a surface of the sheet of solidsubstrate material to form a slurry layer thereon, shearing/compressingand drying the slurry or suspension to form a dried composite precursorlayer deposited on the substrate surface, and collecting compositeprecursor-deposited substrate sheet on a collector roller. The processmay further comprise a step of further compressing the compositeprecursor prior to being collected on the collector roller. This makes aroll-to-roll process amenable to mass production of graphene matrixcomposites.

Alternatively, the process may comprise: (a) preparing a graphene oxidegel having graphene oxide molecules dissolved in a fluid medium to forma homogeneous solution; (b) forming the carbon or graphite filler phaseinto a desired porous shape (e.g. finned heat sink-like shape) havingpores therein, and impregnating the graphene oxide gel into these poresof the desired porous shape to form an impregnated shape under theinfluence of a shear stress; (c) partially or completely removing thefluid medium from the impregnated shape to form a composite precursor;and (d) heat-treating the composite precursor to form the unitarygraphene composite at a temperature higher than 100° C. Again, there-graphitization temperature is preferably >500° C. and morepreferably >1,250° C. Although not required, higher heat treatmenttemperatures may be used if so desired. The desired porous shape may bea porous woven fabric, porous non-woven fabric, porous mat, or porouspaper.

In yet another preferred embodiment, the process for producing theunitary graphene matrix composite comprises: (a) preparing a grapheneoxide gel having graphene oxide molecules dissolved in a fluid medium;(b) combining the carbon or graphite filler phase and the graphene oxidegel to form a graphene oxide gel-impregnated shape of fiber yarns orbundles (e.g. in a finned heat sink shape) wherein the action ofcombining or impregnating is conducted under a shear stress; (c)partially or completely removing the fluid medium from graphene oxidegel-impregnated shape to form a composite precursor; and (d)heat-treating the composite precursor to form the unitary graphenecomposite at a temperature higher than 100° C. The graphene oxidegel-impregnated shape may be selected from a unidirectional,bi-directional, multi-directional, angle-plied, woven, or filament-woundshape.

We have surprisingly observed that the processes for producingconventional resin matrix composites, such as filament winding,pultrusion, and pre-impregnating, may be adapted to fabricate thegraphene matrix composite. The winding, pultrusion, and impregnation canproduce a shear stress field that induces shear thinning and molecularorientations of the nearby GO molecules. This preferred orientation isalso preserved and even enhanced when the resulting GO matrix compositeis dried and heat-treated.

This is quite surprising for several reasons: (1) The GO gel andconventional polymer melts or polymer-solvent solutions appear toexhibit very different and distinct rheological behaviors; (2) It iswell-known in the field of polymer science that highly aromatic chainsare typically not soluble, melt-able, or flowable to enable solution ormelt processing and GO molecules are highly aromatic; (3) Much to thesurprise of polymer scientists, heavy oxidation can chemically convertdiscrete solid graphite flakes to soluble GO molecules and these highlyaromatic molecules can be chemically linked together to form huge 2Dgiant molecules or 3D network of “cross-linked” graphene chains thatprovide cohesiveness and adhesiveness required of a resin matrixcomposite having a good resin-filler interfacial bonding.

The graphene oxide gel may be prepared by immersing a graphitic materialin a powder or fibrous form in an oxidizing liquid to form an initiallyoptically opaque suspension in a reaction vessel at a reactiontemperature for a length of time sufficient to obtain a graphene oxidegel that is optically transparent or translucent. The graphene oxide gelis composed of graphene oxide molecules dispersed in an acidic mediumhaving a typical pH value of no higher than 5 and the graphene oxidemolecules have an oxygen content typically no less than 20% by weightwhen the system is in a gel state.

Specifically, a graphitic material may be immersed in an oxidizing agentto form an optically opaque suspension. It is initially opaque becausethe starting graphitic material is in a carbon or graphite particulateform having a particle size or chemical nature that scatters visiblewavelength or absorbs light. Useful starting materials include naturalgraphite, artificial graphite, meso-phase carbon, meso-phase pitch,meso-carbon micro-bead, soft carbon, hard carbon, coke, carbon fiber,carbon nano-fiber, carbon nano-tube, or a combination thereof. As theoxidizing reaction proceeds to a critical extent, an opticallytransparent or translucent solution is formed.

All the aforementioned processes may further comprise a step ofcompressing the composite precursor prior to or during heat treating.Preferably, the processing conditions involve a shear stress field thatpromotes alignment and/or packing of GO molecules.

The graphene oxide (GO) gel-derived unitary graphene material and theunitary graphene matrix composite have the following characteristics(separately or in combination):

-   (1) The unitary graphene matrix material, alone or with a filler    phase, is an integrated graphene phase that is either a graphene    single crystal or a poly-crystal having multiple grains with    incomplete grain boundaries. When made into a thin film (e.g. <200    μm) or formed under a desired shearing stress field condition, both    the unitary graphene matrix alone or the corresponding graphene    matrix composite have wide/long chemically bonded graphene planes    that are essentially oriented parallel to one another. In other    words, the crystallographic c-axis directions of all grains and all    their constituent graphene planes are essentially pointing in the    same direction. It may be noted that the grains in a graphene    poly-crystal have very poorly delineated or incomplete grain    boundaries. These grains are essentially a single grain with some    residual demarcation lines (e.g., FIG. 3( f)). Such type of graphene    poly-crystal is best described as a graphene single crystal with    some aligned but sporadic defects. These defects can be eliminated    to form a practically perfect single crystal if the unitary graphene    structure is allowed to undergo re-crystallization at a temperature    higher than approximately 2,500° C. for a sufficient length of time.    This conclusion was drawn after an extensive investigation using a    combination of SEM, TEM, selected area diffraction (with a TEM),    X-ray diffraction, atomic force microscopy (AFM), Raman    spectroscopy, and FTIR.-   (2) The paper-like sheets of exfoliated graphite worms (i.e.,    flexible graphite foils), mats of expanded graphite flakes (100 nm    in thickness), and paper or membrane of graphene or GO platelets are    a simple, un-bonded aggregate/stack of multiple discrete graphite    flakes or discrete platelets of graphene, GO, or RGO. In contrast,    the unitary graphene matrix of the present invention is a fully    integrated, single graphene entity or monolith containing no    discrete flakes or platelets derived from the GO gel.-   (3) In prior art processes, discrete graphene sheets (<<100 nm) or    expanded graphite flakes (>100 nm) that constitute the original    structure of graphite particles could be obtained via expanding,    exfoliating, and separating treatments. By simply mixing and    re-compressing these discrete sheets/flakes into a thin film, one    could attempt to orient these sheets/flakes hopefully along one    direction. However, with these conventional processes, the    constituent flakes or sheets of the resulting film (aggregate,    paper, membrane, or mat) would remain as discrete    flakes/sheets/platelets that can be easily discerned or clearly    observed even with an un-assisted eye or under a low-magnification    optical microscope (×100-×1000).

In contrast, the preparation of the presently invented unitary graphenestructure involves heavily oxidizing the original graphite particles, tothe extent that practically every one of the original graphene planeshas been oxidized and isolated from one another to become individualmolecules that possess highly reactive functional groups (e.g. —OH, >0,and —COOH) at the edge and, mostly, on graphene planes as well. Theseindividual hydrocarbon molecules (containing elements such as O and H,in addition to carbon atoms) are dissolved in the reaction medium (e.g.mixture of water and acids) to form a gel-like mass, herein referred toas the GO gel. This gel is then cast onto a smooth substrate surface orinjected into a mold cavity, typically under shear stress fieldconditions, and the liquid components are then removed to form a driedGO layer. When heated, these highly reactive molecules react andchemically join with one another mostly in lateral directions alonggraphene planes (in an edge-to-edge manner) and, in some cases, betweengraphene planes as well. Illustrated in FIG. 3( g) is a plausiblechemical linking mechanism where only 2 aligned GO molecules are shownas an example, although a large number of GO molecules can be chemicallylinked together to form a unitary graphene layer. Further, chemicallinking could also occur face-to-face, not just edge-to-edge. Theselinking and merging reactions proceed in such a manner that themolecules are chemically merged, linked, and integrated into one singleentity or monolith. The molecules completely lose their own originalidentity and they no longer are discrete sheets/platelets/flakes. Thereis only one single layer-like structure (unitary graphene entity) thatis one huge molecule or just a network of interconnected giant moleculeswith an essentially infinite molecular weight. This may also bedescribed as a graphene single crystal (with only one grain in theentire structure or entity, or a poly-crystal (with several grains, buttypically no discernible, well-defined grain boundaries). All theconstituent graphene planes are very large in lateral dimensions (lengthand width) and, if produced under shear stress conditions (particularlyinto thin films, <200 μm in thickness) and heat-treated at a highertemperature (e.g. >1,250° C. or much higher), these graphene planes areessentially parallel to one another.

In-depth studies using a combination of SEM, TEM, selected areadiffraction, X-ray diffraction, AFM, Raman spectroscopy, and FTIRindicate that the graphene monolith is composed of several huge grapheneplanes (with length/width typically >>100 μm, more typically >>1 mm, andmost typically >>1 cm). These giant graphene planes are stacked andbonded along the thickness direction (crystallographic c-axis direction)often through not just the van der Waals forces (as in conventionalgraphite crystallites), but also covalent bonds, Not to be limited bytheory, but Raman and FTIR spectroscopy studies appear to indicate theco-existence of sp² (dominating) and sp³ (weak but existing) electronicconfigurations, not just the conventional sp² in graphite.

-   (4) This integrated graphene entity is not made by gluing or bonding    discrete flakes/platelets together with a resin binder, linker, or    adhesive. Instead, GO molecules in the GO gel are merged through    joining or forming of covalent bonds with one another, into an    integrated graphene entity, without using any externally added    linker or binder molecules or polymers.-   (5) This unitary or monolithic graphene entity is a single crystal    (e.g. FIG. 3( e)) or poly-crystal (having incomplete grain    boundaries, FIG. 3( f)), typically with the crystallographic c-axis    in all grains being essentially parallel to each other. This entity    is derived from a GO gel, which is in turn obtained from natural    graphite or artificial graphite particles originally having multiple    graphite crystallites. Prior to being chemically oxidized, these    starting graphite crystallites have an initial length (L_(a) in the    crystallographic a-axis direction), initial width (L_(b) in the    b-axis direction), and thickness (L_(c) in the c-axis direction).    Upon heavy oxidation, these initially discrete graphite particles    are chemically transformed into highly aromatic graphene oxide    molecules having a significant concentration of edge- or    surface-borne functional groups (e.g. —OH, —COOH, etc.). These    aromatic GO molecules in the GO gel have lost their original    identity of being part of a graphite particle or flake. Upon removal    of the liquid component from the GO gel, the resulting GO molecules    form an essentially amorphous structure. Upon heat treatment    (re-graphitization treatment), these GO molecules are chemically    merged and linked into a unitary or monolithic graphene entity that    is highly ordered (essentially a single crystal when the temperature    is sufficiently high).

The resulting unitary graphene entity typically has a length or widthsignificantly greater than the L_(a) and L_(b) of the originalcrystallites. The length/width of this unitary graphene entity or thatof a graphene single crystal is typically greater than the L_(a) andL_(b) of the original crystallites. Even the individual grains in apoly-crystalline unitary graphene entity have a length or widthsignificantly greater than the L_(a) and L_(b) of the originalcrystallites. They can be as large as the length or width of the unitarygraphene entity itself, not just 2 or 3 times higher than the initialL_(a) and L_(b) of the original crystallites.

-   (6) Due to these unique chemical composition (including oxygen    content), morphology, crystal structure (including inter-graphene    spacing), and structural features (e.g. defects, incomplete or lack    of grain boundaries, chemical bonding and no gap between graphene    sheets, and no interruptions in graphene planes), the graphene oxide    gel-derived unitary or monolithic graphene layer has a unique    combination of outstanding thermal conductivity, electrical    conductivity, mechanical strength, and scratch resistance (including    elimination of the tendency for surface graphite flakes or particles    to “flake off” since there is essentially no GO gel-derived discrete    flake or platelet in this graphene monolith structure). Even in a    unitary graphene matrix composite containing expanded graphite    flakes, these flakes are essentially embraced and bonded with an    integrated graphene film, allowing no exposed flakes.

The aforementioned features are further described and explained indetail as follows: As illustrated in FIG. 1( b), a graphite particle(e.g. 100) is typically composed of multiple graphite crystallites orgrains. A graphite crystallite is made up of layer planes of hexagonalnetworks of carbon atoms. These layer planes of hexagonally arrangedcarbon atoms are substantially flat and are oriented or ordered so as tobe substantially parallel and equidistant to one another in a particularcrystallite. These layers of hexagonal-structured carbon atoms, commonlyreferred to as graphene layers or basal planes, are weakly bondedtogether in their thickness direction (crystallographic c-axisdirection) by weak van der Waals forces and groups of these graphenelayers are arranged in crystallites. The graphite crystallite structureis usually characterized in terms of two axes or directions: the c-axisdirection and the a-axis (or b-axis) direction. The c-axis is thedirection perpendicular to the basal planes. The a- or b-axes are thedirections parallel to the basal planes (perpendicular to the c-axisdirection).

A highly ordered graphite particle can consist of crystallites of aconsiderable size, having a length of L_(a) along the crystallographica-axis direction, a width of L_(b) along the crystallographic b-axisdirection, and a thickness L, along the crystallographic c-axisdirection. The constituent graphene planes of a crystallite are highlyaligned or oriented with respect to each other and, hence, theseanisotropic structures give rise to many properties that are highlydirectional. For instance, the thermal and electrical conductivity of acrystallite are of great magnitude along the plane directions (a- orb-axis directions), but relatively low in the perpendicular direction(c-axis). As illustrated in the upper-left portion of FIG. 1( b),different crystallites in a graphite particle are typically oriented indifferent directions and, hence, a particular property of amulti-crystallite graphite particle is the directional average value ofall the constituent crystallites.

Due to the weak van der Waals forces holding the parallel graphenelayers, natural graphite can be treated so that the spacing between thegraphene layers can be appreciably opened up so as to provide a markedexpansion in the c-axis direction, and thus form an expanded graphitestructure in which the laminar character of the carbon layers issubstantially retained. The process for manufacturing flexible graphiteis well-known and the typical practice is described in U.S. Pat. No.3,404,061 to Shane et al., the disclosure of which is incorporatedherein by reference. In general, flakes of natural graphite (e.g. 100 inFIG. 1( b)) are intercalated in an acid solution to produce graphiteintercalation compounds (GICs, 102). The GICs are washed, dried, andthen exfoliated by exposure to a high temperature for a short period oftime. This causes the flakes to expand or exfoliate in the c-axisdirection of the graphite up to 80-300 times of their originaldimensions. The exfoliated graphite flakes are vermiform in appearanceand, hence, are commonly referred to as worms 104. These worms ofgraphite flakes which have been greatly expanded can be formed withoutthe use of a binder into cohesive or integrated sheets of expandedgraphite, e.g. webs, papers, strips, tapes, foils, mats or the like(typically referred to as “flexible graphite” 106) having a typicaldensity of about 0.04-2.0 g/cm³ for most applications.

The upper left portion of FIG. 1( a) shows a flow chart that illustratesthe prior art processes used to fabricate flexible graphite foils andthe resin-impregnated flexible graphite composite. The processestypically begin with intercalating graphite particles 20 (e.g., naturalgraphite or synthetic graphite) with an intercalant (typically a strongacid or acid mixture) to obtain a graphite intercalation compound 22(GIC). After rinsing in water to remove excess acid, the GIC becomes“expandable graphite.” The GIC or expandable graphite is then exposed toa high temperature environment (e.g., in a tube furnace preset at atemperature in the range of 800-1,050° C.) for a short duration of time(typically from 15 seconds to 2 minutes). This thermal treatment allowsthe graphite to expand in its c-axis direction by a factor of 30 toseveral hundreds to obtain a worm-like vermicular structure 24 (graphiteworm), which contains exfoliated, but un-separated graphite flakes withlarge pores interposed between these interconnected flakes. An exampleof graphite worms is presented in FIG. 2( a).

In one prior art process, the exfoliated graphite (or mass of graphiteworms) is re-compressed by using a calendering or roll-pressingtechnique to obtain flexible graphite foils (26 in FIG. 1( a) or 106 inFIG. 1( b)), which are typically much thicker than 100 μm. An SEM imageof a cross-section of a flexible graphite foil is presented in FIG. 2(b), which shows many graphite flakes with orientations not parallel tothe flexible graphite foil surface and there are many defects andimperfections.

Largely due to these mis-orientations of graphite flakes and thepresence of defects, commercially available flexible graphite foilsnormally have an in-plane electrical conductivity of 1,000-3,000 S/cm,through-plane (thickness-direction or Z-direction) electricalconductivity of 15-30 S/cm, in-plane thermal conductivity of 140-300W/mK, and through-plane thermal conductivity of approximately 10-30W/mK. These defects and mis-orientations are also responsible for thelow mechanical strength (e.g. defects are potential stress concentrationsites where cracks are preferentially initiated). These properties areinadequate for many thermal management applications and the presentinvention is made to address these issues.

In another prior art process, the exfoliated graphite worm 24 may beimpregnated with a resin and then compressed and cured to form aflexible graphite composite 28, which is normally of low strength aswell. In addition, upon resin impregnation, the electrical and thermalconductivity of the graphite worms could be reduced by two orders ofmagnitude.

Alternatively, the exfoliated graphite may be subjected tohigh-intensity mechanical shearing/separation treatments using ahigh-intensity air jet mill, high-intensity ball mill, or ultrasonicdevice to produce separated nano graphene platelets 33 (NGPs) with allthe graphene platelets thinner than 100 nm, mostly thinner than 10 nm,and, in many cases, being single-layer graphene (also illustrated as 112in FIG. 1( b). An NGP is composed of a graphene sheet or a plurality ofgraphene sheets with each sheet being a two-dimensional, hexagonalstructure of carbon atoms.

Further alternatively, with a low-intensity shearing, graphite wormstend to be separated into the so-called expanded graphite flakes (108 inFIG. 1( b) having a thickness >100 nm. These flakes can be formed intographite paper or mat 106 using a paper- or mat-making process. Thisexpanded graphite paper or mat 106 is just a simple aggregate or stackof discrete flakes having defects, interruptions, and mis-orientationsbetween these discrete flakes.

For the purpose of defining the geometry and orientation of an NGP, theNGP is described as having a length (the largest dimension), a width(the second largest dimension), and a thickness. The thickness is thesmallest dimension, which is no greater than 100 nm, preferably smallerthan 10 nm in the present application. When the platelet isapproximately circular in shape, the length and width are referred to asdiameter. In the presently defined NGPs, both the length and width canbe smaller than 1 μm, but can be larger than 200 μm.

A mass of multiple NGPs (including discrete sheets/platelets ofsingle-layer and/or few-layer graphene, 33 in FIG. 1( a)) may be madeinto a graphene film/paper (34 in FIG. 1( a) or 114 in FIG. 1( b)) usinga film- or paper-making process. FIG. 3( b) shows a SEM image of across-section of a graphene paper/film prepared from discrete graphenesheets using a paper-making process. The image shows the presence ofmany discrete graphene sheets being folded or interrupted (notintegrated), most of platelet orientations being not parallel to thefilm/paper surface, the existence of many defects or imperfections. NGPaggregates, even when being closely packed, exhibit a thermalconductivity higher than 1,000 W/mK only when the film or paper is castand strongly pressed into a sheet having a thickness lower than 10 μm. Aheat spreader in many electronic devices or a fin member in a heat sinkis normally required to be thicker than 25 μm and, more desirably,thicker than 50 μm based mainly on handling ease and structuralintegrity considerations (but no greater than 200 μm due to devicevolume constraint).

The precursor to the unitary graphene layer is graphene oxide gel 21(FIG. 1( a)). This GO gel is obtained by immersing a graphitic material20 in a powder or fibrous form in a strong oxidizing liquid in areaction vessel to form a suspension or slurry, which initially isoptically opaque and dark. This optical opacity reflects the fact that,at the outset of the oxidizing reaction, the discrete graphite flakesand, at a later stage, the discrete graphene oxide flakes scatter and/orabsorb visible wavelengths, resulting in an opaque and generally darkfluid mass. If the reaction between graphite powder and the oxidizingagent is allowed to proceed at a sufficiently high reaction temperaturefor a sufficient length of time, this opaque suspension is transformedinto a brown-colored and typically translucent or transparent solution,which is now a homogeneous fluid called “graphene oxide gel” (21 in FIG.1( a)) that contains no discernible discrete graphite flakes or graphiteoxide platelets. If dispensed and deposited under a shear stress field,the GO gel undergoes viscosity reduction and molecular orientation toform “oriented GO” 35, which can be heat-treated to become a unitarygraphene material 37.

Again, this graphene oxide gel is typically optically transparent ortranslucent and visually homogeneous with no discernible discreteflakes/platelets of graphite, graphene, or graphene oxide dispersedtherein. In the GO gel, the GO molecules are uniformly dissolved in anacidic liquid medium. In contrast, conventional suspension of discretegraphene sheets, graphene oxide sheets, and expanded graphite flakes ina fluid (e.g. water, organic acid or solvent) look dark, black or heavybrown in color with individual graphene or graphene oxide sheets orexpanded graphite flakes discernible or recognizable even with nakedeyes or a low-magnification light microscope (100×-1,000×).

The graphene oxide molecules dissolved in the liquid medium of agraphene oxide gel are aromatic chains that have an average number ofbenzene rings in the chain typically less than 1,000, more typicallyless than 500, and many less than 100. Most of the molecules have morethan 5 or 6 benzene rings (mostly >10 benzene rings) from combinedatomic force microscopy, high-resolution TEM, and molecular weightmeasurements. Based on our elemental analysis, these benzene-ring typeof aromatic molecules are heavily oxidized, containing a highconcentration of functional groups, such as —COOH and —OH and,therefore, are “soluble” (not just dispersible) in polar solvents, suchas water. The estimated molecular weight of these graphene oxidepolymers in the gel state is typically between 200 g/mole and 43,000g/mole, more typically between 400 g/mole and 21,500 g/mole, and mosttypically between 400 g/mole and 4,000 g/mole.

These soluble molecules behave like polymers and are surprisinglycapable of reacting and getting chemically connected with one another(during the subsequent heat treatment or re-graphitization treatment) toform a unitary graphene layer of good structural integrity and highthermal conductivity. Conventional discrete graphene sheets, grapheneoxide sheets, or graphite flakes do not have any self-reacting orcohesive bonding capability. Also very surprisingly, during thesubsequent heat treatment or re-graphitization treatment, these solublemolecules in the GO gel are capable of chemically bonding a carbon orgraphite filler phase (e.g. carbon fibers, expanded graphite flakes,CNTs, carbon black particles, etc.) dispersed in the GO gel.

Again, specifically and most significantly, these graphene oxidemolecules present in a GO gel state are capable of chemically bonding,linking, or merging with one another and getting integrated intoextremely long and wide graphene layers (e.g. FIG. 3( a)) when the gelis dried and heat-treated at a sufficiently high temperature for asufficiently long period of time. These graphene layers can run as wideas the specimen width itself (up to hundreds of centimeters) that areparallel to one another. No individual graphene platelets or sheets arediscernible; they have been chemically converted to chemically active orlive GO molecules that are fully linked and integrated chemically withone another to form a layer-like unitary body in the graphene planedirection and these unitary bodies appear to be chemically bonded withone another along the thickness-direction (or Z-direction). X-raydiffraction studies have confirmed that the d-spacing (inter-grapheneplane distance) has been recovered back to approximately 0.3354 nm (with0%-0.001% by weight of oxygen) to 0.40 nm (with approximately 5.0-10%oxygen). There does not appear to be any gap between these graphenelayers and, hence, these layers have been essentially merged into onebig unitary body, which is a graphene single crystal. FIG. 3( a) depictsan example of such a huge unitary body. Although there appears to besome demarcations between unitary layers, these perceived demarcationsare due to slightly different widths between layers. Each layer iscomposed of one of multiple graphene planes parallel to one another.These seemingly individual unitary layers actually have formed into asingle integrated entity or a graphene single crystal. The formationprocess for such a graphene single crystal is further illustrated inFIG. 3( c).

It may be noted that the presently invented graphene single crystal isfundamentally different and patently distinct from the catalytic CVDgraphene thin film in terms of chemical composition, micro-structure,morphology, process of production, all chemical and physical properties,and intended applications. This is explained as follows:

-   (a) As schematically shown in FIG. 3( d), the prior art graphene    poly-crystal obtained by CVD of hydrocarbon on a catalytic surface    (e.g. Cu or Ni) is typically composed of many grains with grain size    typically smaller than 10 μm (most often <5 μm). These grains also    have different orientations with respect to one another. The CVD    graphene contains many defects, e.g., grain boundaries, line    defects, vacancies, and other lattice defects, such as those many    carbon atoms configured in pentagons, heptagons, or octagons, as    opposed to the normal hexagon. These defects impede the flow of    electrons and phonons.-   (b) In contrast, FIG. 3( e) shows a schematic of a graphene single    crystal of the present invention having just one single grain or    domain. There are no grain boundaries that can impede the movement    of electrons or phonons and, hence, this single-grain single-crystal    material has an exceptionally high electrical conductivity and    thermal conductivity.-   (c) FIG. 3( f) shows a schematic of another graphene single crystal    of the present invention, which is a “poly-crystal” with incomplete    grain boundaries. The graphene planes in all the grains are oriented    parallel to one another.-   (d) The presently invented graphene single crystal from GO gel can    have some oxygen content, but no hydrogen (H). In contrast, the    catalytic CVD graphene film inherently has some hydrogen or    nitrogen, but no oxygen.-   (e) Typically, the CVD graphene film grown on Cu or Ni surface is    single layer or inhomogeneous few-layer graphene with a thickness    less than 2 nm (the underlying Cu or Ni foil is not capable of    providing catalytic effect when the deposited carbon layer exceeds 2    nm). These ultra-thin layers are thus optically transparent and are    intended to replace the ITO glass for use in touch panel screens.    These ultra-thin films are not rigid enough to easily handled and    implemented as a heat spreader in a smart phone. In contrast, our    graphene monolith is typically thicker than 10 nm (more typically    thicker than 100 nm and most typically thicker than 10 μm) and,    hence, is optically opaque. The graphene monolith of the present    invention has a significantly higher thermal conductivity and can be    more easily handled when being implemented into an electronic device    (e.g. a mobile phone) as a heat spreader, or made into a finned heat    sink for a LED lighting device.-   (f) The electrical conductivity (<1,000 S/cm) and thermal    conductivity (<500 W/mK) of the CVD graphene films are typically    significantly lower than those of the presently invented graphene    single crystals even though these CVD films are typically thinner    than 2 nm and our graphene single crystals are typically thicker    than 10 nm (often thicker than 10 μm).

The starting graphitic material to be heavily oxidized for the purposeof forming graphene oxide gel may be selected from natural graphite,artificial graphite, meso-phase carbon, meso-phase pitch, meso-carbonmicro-bead, soft carbon, hard carbon, coke, carbon fiber, carbonnano-fiber, carbon nano-tube, or a combination thereof. The graphiticmaterial is preferably in a powder or short filament form having adimension lower than 20 μm, more preferably lower than 10 μm, furtherpreferably smaller than 5 μm, and most preferably smaller than 1 μM.

Using artificial graphite with an average particle size of 9.7 μm as anexample, a typical procedure involves dispersing graphite particles inan oxidizer mixture of sulfuric acid, nitric acid, and potassiumpermanganate (at a weight ratio of 3:1:0.05) at a temperature oftypically 0-60° C. for typically at least 3 days, preferably 5 days, andmore preferably 7 days or longer. The average molecular weight of theresulting graphene oxide molecules in a gel is approximately20,000-40,000 g/mole if the treatment time is 3 days, <10,000 g/mole if5 days, and <4,000 g/mole if longer than 7 days. The required gelformation time is dependent upon the particle size of the originalgraphitic material, a smaller size requiring a shorter time. It is offundamental significance to note that if the critical gel formation timeis not reached, the suspension of graphite powder and oxidizer (graphiteparticles dispersed in the oxidizer liquid) appears completely opaqueand heterogeneous, meaning that discrete graphite particles or flakesremain suspended (but not dissolved) in the liquid medium. As soon asthis critical time is exceeded, the whole suspension becomes opticallytranslucent or transparent (if sufficiently low GO contents) and browncolored, meaning that the heavily oxidized graphite completely loses itsoriginal graphite identity and the resulting graphene oxide moleculesare completely dissolved in the oxidizer liquid, forming a homogeneoussolution (no longer just a suspension or slurry).

It must be further noted that if the suspension or slurry, with atreatment time being shorter than the required gel formation time, isrinsed and dried, we would simply recover a graphite oxide powder orgraphite intercalation compound (GIC) powder, which can be exfoliatedand separated to produce discrete nano graphene platelets (NGPs).Without an adequate amount of a strong oxidizing agent and an adequateduration of oxidation time, the graphite or graphite oxide particleswould not be converted into the GO gel state.

The graphene oxide-derived unitary graphene matrix composite containinga carbon or graphite filler phase of the present invention typically hasa thermal conductivity greater than 800 W/mK, more typically greaterthan 1,000 W/mK (even when the film thickness is greater than 10 μm) andoften greater than 1,700 W/mK. This latter valve is typically obtainedwhen the carbon/graphite filler is exfoliated graphite flakes (>100 nm,but preferably <500 nm) or pristine graphene platelets (<100 nm,preferably <10 nm) and when the final heat treatment temperature ishigher than 2,500° C. The graphene matrix composite typically has anelectrical conductivity greater than 3,000 S/cm (even >10,000 S/cm).This high electrical conductivity (greater than 3000 S/cm and up to15,000 S/cm) can be achieved concurrently with a thermal conductivitygreater than 1,000 W/mK (up to 1,800 W/mK). Quite often, the unitarygraphene matrix composite can exhibit a combination of a high electricalconductivity (greater than 1,500 S/cm, more often >3,000 S/cm), a highthermal conductivity (greater than 600 W/mK, more often greater than 800W/mK), a relatively high physical density (greater than 1.8 g/cm³), anda relatively high tensile strength (greater than 40 MPa, often >80 MPa,and can be >120 MPa). Unidirectional carbon fiber reinforced graphenematrix composites can exhibit a tensile strength significantly higherthan 200 MPa. The unitary graphene matrix composite also exhibits anexceptional surface hardness and scratch resistance, eliminating thetendency to flake off (to emit free carbon or graphite particles intoair) which has been a serious problem associated with the flexiblegraphite foil and the recompressed graphene platelet foil.

If he graphene oxide gel is obtained from a graphitic material having anoriginal graphite grain size (e.g. an average grain size, D_(g)), theresulting unitary graphene material is a single crystal or apoly-crystal graphene structure having a grain size significantly largerthan this original grain size. The unitary graphene material does nothave any grain that can be identified to be associated with anyparticular particle of the starting graphitic material. Originalparticles have already completely lost their identity when they areconverted into graphite oxide molecules that are chemically linked upand merged or integrated into a network of graphene chains essentiallyinfinite in molecular weight.

Further, even if graphene oxide gel is obtained from a graphiticmaterial having multiple graphite crystallites exhibiting no preferredcrystalline orientation (e.g. powder of natural graphite) as determinedby an X-ray diffraction or electron diffraction method, the resultingunitary graphene material (a single crystal or a poly-crystal graphenestructure) typically exhibits a very high degree of preferredcrystalline orientation as determined by the same X-ray diffraction orelectron diffraction method. This is yet another piece of evidence toindicate that the constituent graphene planes of hexagonal carbon atomsthat constitute the particles of the original or starting graphiticmaterial have been chemically modified, converted, re-arranged,re-oriented, linked or cross-linked, merged and integrated,re-graphitized, and even re-crystallized.

Example 1 Preparation of Discrete Nano Graphene Platelets (NGPs) andExpanded Graphite Flakes

Chopped graphite fibers with an average diameter of 12 μm and naturalgraphite particles were separately used as a starting material, whichwas immersed in a mixture of concentrated sulfuric acid, nitric acid,and potassium permanganate (as the chemical intercalate and oxidizer) toprepare graphite intercalation compounds (GICs). The starting materialwas first dried in a vacuum oven for 24 h at 80° C. Then, a mixture ofconcentrated sulfuric acid, fuming nitric acid, and potassiumpermanganate (at a weight ratio of 4:1:0.05) was slowly added, underappropriate cooling and stirring, to a three-neck flask containing fibersegments. After 16 hours of reaction, the acid-treated graphite fibersor natural graphite particles were filtered and washed thoroughly withdeionized water until the pH level of the solution reached 6. Afterbeing dried at 100° C. overnight, the resulting graphite intercalationcompound (GIC) was subjected to a thermal shock at 1050° C. for 45seconds in a tube furnace to form exfoliated graphite (or graphiteworms).

Five grams of the resulting exfoliated graphite (graphite worms) weremixed with 2,000 ml alcohol solution consisting of alcohol and distilledwater with a ratio of 65:35 for 12 hours to obtain a suspension. Thenthe mixture or suspension was subjected to ultrasonic irradiation with apower of 200 W for various times. After two hours of sonication, EGparticles were effectively fragmented into thin NGPs. The suspension wasthen filtered and dried at 80° C. to remove residue solvents. Theas-prepared NGPs have an average thickness of approximately 9.7 nm.

Another five grams of the resulting exfoliated graphite (EG) weresubjected to low-intensity air jet milling to break up graphite worms,forming expanded graphite flakes (having an average thickness of 139nm).

Example 2 Preparation of Single-Layer Graphene Sheets from Meso-CarbonMicro-Beads MCMBs

Meso-carbon microbeads (MCMBs) were supplied from China Steel ChemicalCo. This material has a density of about 2.24 g/cm³ with a medianparticle size of about 16 μm. MCMB (10 grams) were intercalated with anacid solution (sulfuric acid, nitric acid, and potassium permanganate ata ratio of 4:1:0.05) for 72 hours. Upon completion of the reaction, themixture was poured into deionized water and filtered. The intercalatedMCMBs were repeatedly washed in a 5% solution of HCl to remove most ofthe sulphate ions. The sample was then washed repeatedly with deionizedwater until the pH of the filtrate was neutral. The slurry was dried andstored in a vacuum oven at 60° C. for 24 hours. The dried powder samplewas placed in a quartz tube and inserted into a horizontal tube furnacepre-set at a desired temperature, 1,080° C. for 45 seconds to obtain agraphene material. TEM and atomic force microscopic studies indicatethat most of the NGPs were single-layer graphene.

Example 3 Preparation of Pristine Graphene Sheets/Platelets

In a typical procedure, five grams of graphite flakes, ground toapproximately 20 μm or less in sizes, were dispersed in 1,000 mL ofdeionized water (containing 0.1% by weight of a dispersing agent, Zonyl®FSO from DuPont) to obtain a suspension. An ultrasonic energy level of85 W (Branson S450 Ultrasonicator) was used for exfoliation, separation,and size reduction of graphene sheets for a period of 15 minutes to 2hours.

Example 4 Preparation of Graphene Oxide (GO) Gel

Graphite oxide gel was prepared by oxidation of graphite flakes with anoxidizer liquid consisting of sulfuric acid, sodium nitrate, andpotassium permanganate at a ratio of 4:1:0.05 at 30° C. When naturalgraphite flakes (particle sizes of 14 μm) were immersed and dispersed inthe oxidizer mixture liquid, the suspension or slurry appears opticallyopaque and dark. The suspension remains opaque during the first 52 hoursof reaction. However, the suspension gradually turns opticallytranslucent (a little cloudy) when the reaction time exceeds 52 hours,and the color of the suspension changes from black to dark brown. After96 hours, the suspension suddenly becomes an optically transparentsolution with light brown color. The solution appears very uniform incolor and transparency, indicating the absence of any dispersed discreteobjects. The whole solution behaves like a gel, very similar to atypical polymer gel.

Surprisingly, by casting this gel on a glass surface and removing theliquid medium from the cast film we obtain a thin film of graphene oxidethat is optically transparent. This thin film looks like, feels like,and behaves like a regular polymer film. However, upon re-graphitizationat a temperature (typically >100° C., more typically >500° C., furthertypically >1,250° C., and can be >2,500° C.) for typically 1-3 hours,this GO film is transformed into a unitary graphene entity comprising orbeing a large-size graphene single crystal. This is a free-standingunitary graphene layer, which can be implemented directly as a heatspreader in an electronic device or used as a matrix material in agraphene matrix composite containing a carbon/graphite filler phase.

X-ray diffraction curves of a GO film (GO gel coated on a glass surfacewith liquid medium removed) prior to a heat treatment, a GO filmthermally reduced at 150° C. for one hour, and a highly reduced andre-graphitized GO film (a unitary graphene layer) are shown in FIGS. 5(a), 5(b), and 5(c), respectively. The peak at approximately 20=12° ofthe dried GO film (FIG. 5( a)) corresponds to an inter-graphene spacing(d₀₀₂) of approximately 0.7 nm. With some heat treatment at 150° C., theGO film exhibits the formation of a hump centered at 22° (FIG. 5( b)),indicating that it has begun the process of decreasing theinter-graphene spacing, indicating the beginning of chemical linking andordering processes. With a heat treatment temperature of 2,500° C. forone hour, the d₀₀₂ spacing has decreased to approximately 0.336, closeto 0.3354 nm of a graphite single crystal.

With a heat treatment temperature of 2,750° C. for one hour, the d₀₀₂spacing is decreased to approximately to 0.3354 nm, identical to that ofa graphite single crystal. In addition, a second diffraction peak with ahigh intensity appears at 20=55° corresponding to X-ray diffraction from(004) plane (FIG. 5( d)). The (004) peak intensity relative to the (002)intensity on the same diffraction curve, or the I(004)/I(002) ratio, isa good indication of the degree of crystal perfection and preferredorientation of graphene planes. The (004) peak is either non-existing orrelatively weak, with the I(004)/I(002) ratio <0.1, for all graphiticmaterials heat treated at a temperature lower than 2,800° C. TheI(004)/I(002) ratio for the graphitic materials heat treated at3,000-3,250° C. (e.g, highly oriented pyrolytic graphite, HOPG) is inthe range of 0.2-0.5. One example is presented in FIG. 5( e) for apolyimide-derived PG with a HTT of 3,000° C. for two hours, whichexhibits a I(004)/I(002) ratio of about 0.41. In contrast, a unitarygraphene single crystal prepared with a HTT of 2,750° C. for one hourexhibits a I(004)/I(002) ratio of 0.78 and a Mosaic spread value of0.21, indicating a practically perfect graphene single crystal with anexceptional degree of preferred orientation.

The “mosaic spread” value obtained from the full width at half maximumof the (002) reflection in an X-ray diffraction intensity curve. Thisindex for the degree of ordering characterizes the graphite or graphenecrystal size (or grain size), amounts of grain boundaries and otherdefects, and the degree of preferred grain orientation. A nearly perfectsingle crystal of graphite is characterized by having a mosaic spreadvalue of 0.2-0.4. Most of our unitary graphene materials have a mosaicspread value in this range of 0.2-0.4 (with a heat treatment temperatureno less than 2,000° C.).

It may be noted that the I(004)/I(002) ratio for all tens of flexiblegraphite samples investigated are all <<0.05, practically non-existingin most cases. The I(004)/I(002) ratio for all NGP paper/membranesamples is <0.1 even after a heat treatment at 3,000° C. for 2 hours.Attempts to graphitize the ultra-thin films (<2 nm in thickness)prepared by Cu-catalyzed CVD led to the breaking up of the film and theformation of graphite particles instead. These observations have furtherconfirmed or affirmed the already established notion that the presentlyinvented unitary graphene crystal is a new and distinct class ofmaterial that is fundamental different from any pyrolytic graphite (PG),flexible graphite (FG), and paper/film/membrane of conventionalgraphene/GO/RGO sheets/platelets (NGPs).

The inter-graphene spacing values of GO gel-derived unitary graphenefilms obtained by heat treating at various temperatures over a widetemperature range are summarized in FIG. 6( a). Corresponding oxygencontent values in the GO gel-derived unitary graphene layer are shown inFIG. 6( b). In order to show the correlation between the inter-graphenespacing and the oxygen content, the data in FIGS. 6( a) and 6(b) arere-plotted in FIG. 6( c). A close scrutiny of FIG. 69( a)-(c) indicatethat there are four HTT ranges (100-500° C.; 500-1,250° C.; 1,250-2,000°C., and >2,000° C.) that lead to four respective oxygen content rangesand inter-graphene spacing range. The thermal conductivity of GOgel-derived unitary graphene layer and corresponding flexible graphite(FG) foil, also plotted as a function of the same final heat treatmenttemperature range is summarized in FIG. 6( d).

It is of significance to point out that a heat treatment temperature aslow as 500° C. is sufficient to bring the average inter-graphene spacingin GO to below 0.4 nm, getting closer and closer to that of naturalgraphite or that of a graphite single crystal. The beauty of thisapproach is the notion that this GO gel strategy has enabled us tore-organize, re-orient, and chemically merge the planar graphene oxidemolecules from originally different graphite particles or graphenesheets into a graphene monolith with all the graphene planes now beinglarger in lateral dimensions (significantly larger than the length andwidth of original graphene planes) and essentially parallel to oneanother. This has given rise to a thermal conductivity already >420 W/mK(with a HTT of 500° C.) and >950 W/mk with a HTT of 700° C.), which ismore than 2- to 4-fold greater than the value (200 W/mK) of thecorresponding flexible graphite foil. These planar GO molecules arederived from the graphene planes that constitute the original structureof starting natural graphite particles (used in the procedure ofgraphite oxidation to form the GO gel). The original natural graphiteparticles, when randomly packed into an aggregate or “graphite compact”,would have their constituent graphene planes randomly oriented,exhibiting relatively low thermal conductivity and having essentiallyzero strength (no structural integrity). In contrast, the strength ofthe unitary graphene layer (even without an added reinforcement) istypically already in the range of 40-140 MPa.

With a HTT as low as 800° C., the resulting unitary graphene layerexhibits a thermal conductivity of 1,148 W/mK, in contrast to theobserved 244 W/mK of the flexible graphite foil with an identical heattreatment temperature. As a matter of fact, no matter how high the HTTis (e.g. even as high as 2,800° C.), the flexible graphite foil onlyshows a thermal conductivity lower than 600 W/mK. At a HTT of 2,800° C.,the presently invented unitary graphene layer delivers a thermalconductivity of 1,807 W/mK (FIG. 4( a) and FIG. 6( d)).

Scanning electron microscopy (SEM), transmission electron microscopy(TEM) pictures of lattice imaging of the graphene layer, as well asselected-area electron diffraction (SAD), bright field (BF), anddark-field (DF) images were also conducted to characterize the structureof unitary graphene materials. For measurement of cross-sectional viewsof the film, the sample was buried in a polymer matrix, sliced using anultra-microtome, and etched with Ar plasma.

A close scrutiny and comparison of FIGS. 2( a), 3(a), and 3(b) indicatesthat the graphene layers in a graphene single crystal or graphenemonolithic are substantially oriented parallel to one another; but thisis not the case for flexible graphite foils and graphene oxide paper.The inclination angles between two identifiable layers in the unitarygraphene entity are mostly less than 5 degrees. In contrast, there areso many folded graphite flakes, kinks, and mis-orientations in flexiblegraphite that many of the angles between two graphite flakes are greaterthan 10 degrees, some as high as 45 degrees (FIG. 2( b)). Although notnearly as bad, the mis-orientations between graphene platelets in NGPpaper (FIG. 3( b)) are also high and there are many gaps betweenplatelets. The unitary graphene entity is essentially gap-free.

FIG. 4 (a) shows the thermal conductivity values of the GO gel-derivedunitary graphene matrix layer (▴), GO platelet paper (▪) prepared byvacuum-assisted filtration of RGO, and FG foil (♦), respectively, allplotted as a function of the final HTT for graphitization orre-graphitization. These data have clearly demonstrated the superiorityof the unitary graphene material or graphene single crystal in terms ofthe achievable thermal conductivity at a given heat treatmenttemperature. All the prior art work on the preparation of paper ormembrane from pristine graphene or graphene oxide sheets/plateletsfollows distinctly different processing paths, leading to a simpleaggregate or stack of discrete graphene/GO/RGO platelets. These simpleaggregates or stacks exhibit many folded graphite flakes, kinks, gaps,and mis-orientations, resulting in poor thermal conductivity, lowelectrical conductivity, and weak mechanical strength. As shown in FIG.4( a), even at a heat treatment temperature as high as 2,800° C., the GOplatelet paper exhibits a thermal conductivity less than 1,000 W/mK,much lower than the >1,800 W/mK of the GO gel-derived unitary grapheneentity.

For comparison, we have also carbonized polyimide films at 500° C. for 1hour and at 1,000° C. for 3 hours in an inert atmosphere and thengraphitized the films at a temperature in the range of 2,500-3,000° C.for 1 to 5 hours to form a conventional pyrolytic graphite (PG) film.FIG. 4( b) shows the thermal conductivity values of the GO-derivedunitary graphene (▪) and the polyimide-derived PG heat-treated for onehour (x) and for 3 hours (▴), all plotted as a function of the finalgraphitization or re-graphitization temperature. These data show thatthe conventional PG, produced by carbonizing polyimide (PI) and thengraphitizing the carbonized PI, exhibits a consistently lower thermalconductivity as compared to the GO gel-derived unitary graphene alone(▪), given the same HIT for the same length of heat treatment time. Forinstance, the PG from PI exhibits a thermal conductivity of 820 W/mKafter a graphitization treatment at 2,000° C. for one hour and 1,242W/mK at 2,000° C. for 3 hours. These observations have demonstrated aclear and significant advantage of using the GO gel approach toproducing unitary graphene materials versus the conventional PG approachto producing oriented graphite crystals. As a matter of fact, no matterhow long the graphitization time is for the PG, the thermal conductivityis always lower than that of a GO gel-derived unitary graphene. In otherwords, the unitary graphene material is fundamentally different andpatently distinct from the flexible graphite (FG) foil, graphene/GO/RGOpaper/membrane, and pyrolytic graphite (PG) in terms of chemicalcomposition, crystal and defect structure, crystal orientation,morphology, process of production, and properties.

The above conclusion is further supported by the data in FIG. 4( c)showing the electric conductivity values of the GO-derived unitarygraphene layer (♦) are far superior to those of the GO paper (▪) fromRGO platelets and FG foil (x) over the entire range of final HTTsinvestigated.

Examples 5 Preparation and Testing of Unitary Graphene Matrix Composites

GO gel can be combined with a carbon/graphite filler phase to form agraphene matrix composite. The graphene oxide gel prepared in Example 4was used for the preparation of graphene matrix composite. Theexfoliated graphite flakes prepared in Examples 1 were made into thinporous paper or film form (e.g., using a vacuum-assisted filtrationtechnique) for use as a carbon/graphite filler. Other carbon or graphitefillers investigated include carbon nano-tubes and CNT paper (Buckypaper from Buckeye Composites, Inc., Dayton, Ohio), carbon nano-fibersand CNF mats (CNFs supplied from Applied Sciences, Inc., Cedarville,Ohio), flexible graphite foils of several different thicknesses(supplied from Graftech and Timcal Graphite), carbon fibers and carbonfiber mats, woven fabrics of graphite fibers, carbon paper (Toray), MCMBparticles, carbon black (CB), acetylene black (AB), and needle coke.

As examples, two approaches were adapted to produce graphene matrixcomposites. In the first approach, the particles of the carbon/graphitefiller phase were formed into porous pre-forms, such as porous paper,mat, and fabric (woven or non-woven). The porous pre-form was thenimpregnated with GO gel, which was followed by drying and heat treating.

In a second approach, discrete particles or fibers of thecarbon/graphite filler phase were added into the GO gel to form amixture gel or gel slurry. Pure GO gel or carbon/graphite filler-GOmixture gel or slurry was then cast onto a solid substrate surface usinga coating machine equipped with drying and heating provisions. In somecases, the GO gel or filler-GO gel mixture was cast onto a substrate andregulated by a doctor's blade to form a uniform coating thereon. Thisprocedure creates a shear stress field that induces viscosity thinningand molecular orientation. The liquid in the coating was further removedin a vacuum oven to form a solid GO coating. The resulting GO orGO-filler layers were then subjected to a heat treatment at atemperature of from 100° C. up to approximately 3,000° C. We haveutilized several temperature regimes: 100° C.-500° C.; 500° C.-1,250°C.; 1,250° C.-2,000° C.; and 2,000° C.-3,000° C.

Examples 6 Electrical and Thermal Conductivity Measurements of VariousGraphene Oxide-Derived Unitary Graphene and Graphene Matrix CompositeLayers

Four-point probe tests were conducted on unitary graphene matrixcomposites (e.g. containing CNT, expanded graphite flakes, carbon black,etc), the GO gel-derived unitary graphene layer alone (coated on a glasssurface and then peeled off and heat treated), GO/RGO paper, and the FGfoils alone to measure their in-plane electrical conductivity. Theirin-plane thermal conductivity was measured using a laser flash method(Netzsch Thermal Diffusivity Device).

The in-plane thermal and electrical conductivities and tensileproperties of various films or laminates were investigated. Severalsignificant observations can be made from the testing results (e.g. assummarized in FIGS. 4( d), 4(e), 7(a), 7(b), 8(a), and 8(b)):

-   (1) With a thickness of approximately 75 μm, the thermal    conductivity of the flexible graphite foil alone (FG, ▴ in FIG. 4(    a)) is less than 237 W/mK if the FG foil is not heat-treated at or    above 700° C. As the post-recompression heat treatment temperature    increases from 700° C. to 2,800° C. (for one hour of graphitization    treatment in each case), the thermal conductivity of the FG foil    increases from 237 to 582 W/mK, indicating some but limited    re-organization of the graphitic structure induced by the heat    treatment. By contrast, the thermal conductivity of the GO    gel-derived unitary graphene layer alone increases from 983 to 1,807    W/mK (▪ in FIG. 7( a)). This unitary graphene matrix material is    obtained by shearing and depositing a layer of GO gel on a glass    surface, removing the liquid from the GO layer in vacuum for 1 hour,    and peeling off the dried solid GO layer from the glass surface.    This indicates a significant or dramatic re-organization of the    graphitic structure induced by the heat treatment, with all GO    molecules linked or merged edge-to-edge and face-to-face into a    unitary graphene body of fully and orderly bonded graphene planes, a    graphene single crystal.-   (2) The experimentally measured thermal conductivity of a    corresponding series of GO gel-derived unitary graphene matrix    composite containing expanded graphite flakes as the filler phase (♦    in FIG. 7( a)) increases from approximately 800 to 1,800 W/mK. This    is significantly higher than the thermal conductivity values of what    would be theoretically predicted (x in FIG. 7 a)) from a    rule-of-mixture law, which is commonly used to predict composite    properties from constituent properties. These data have clearly    demonstrated an un-expected, synergistic effect between GO    gel-derived unitary graphene matrix (derived from graphene oxide    gel) and the dispersed expanded graphite flakes.

Also shown in FIG. 7( a) are the thermal conductivity data ofcorresponding flexible graphite foil (FG prepared by roll-pressing ofexfoliated graphite worms) and foil or mat of expanded graphite flakes(prepared by breaking up graphite worms into graphite flakes asdescribed in Example 1, which were then packed and roll-pressed into athin foil/mat). The highest thermal conductivity value achievable withthe expanded graphite foil is <800 W/mK and that with FG is <600 W/mK,both being dramatically lower than those of both the unitary graphenematrix and the graphene matrix composite.

-   (3) FIG. 7( b) shows that the conventional PG, produced by    carbonizing polyimide, roll-pressing, and then graphitizing the    carbonized PI, exhibits a consistently lower thermal conductivity as    compared to the GO gel-derived unitary graphene layer alone (▪) or    unitary graphene matrix composite (♦), given the same HTT for the    same length of heat treatment time. For instance, the PG from PI    exhibits a thermal conductivity of 820 W/mK after a graphitization    treatment at 2,000° C. for one hour and 1,242 W/mK at 2,000° C. for    3 hours. These observations have demonstrated a clear and    significant advantage of using the GO gel approach versus the    conventional PG approach. As a matter of fact, no matter how long    the graphitization time is for the PG, the thermal conductivity is    always lower than that of a GO gel-derived unitary graphene or    unitary graphene matrix composite. These observations have clearly    further validate the notion that both the GO gel-derived unitary    graphene layer and unitary graphene matrix composite are    fundamentally different and patently distinct from the pyrolytic    graphite in terms of chemical composition, structure, morphology,    process of production, and properties.-   (4) FIG. 4( d) shows the thermal conductivity values of both unitary    graphene matrix and graphene matrix-CNT composite are far superior    to those of prior art GO platelet paper containing discrete GO    platelets and those of GO platelet paper containing an equal    proportion of the same CNTs (approximately 26% by weight). FIG. 4(    e) demonstrates that unitary graphene matrix composite containing    carbon black particles as the carbon/graphite filler phase are    significantly higher than those of prior art GO paper and    corresponding GO-CB paper.

Examples 7 Tensile Strength of Various Graphene Oxide-Derived UnitaryGraphene Matrix Composites

A series of GO gel-derived unitary graphene layers, graphene matrixcomposites, GO platelet paper, and FG foil were prepared. A universaltesting machine was used to determine the tensile strength of thesematerials. The tensile strength values of the unitary graphene entity,GO platelet paper, and FG paper are plotted as a function of there-graphitization temperature, FIG. 8( a). These data have demonstratedthat the tensile strength of the flexible graphite foil remainsrelatively constant (all <20 MPa) and that of the GO paper increasesslightly (from 22 to 43 MPa) when the heat treatment temperatureincreases from 700 to 2,800° C. In contrast, the tensile strength of theGO-derived unitary graphene layer increases dramatically from 32 to >100MPa over the same range of heat treatment temperatures. This result isquite striking and further reflects the notion that the GO gel-derivedGO layer contains highly live and active molecules during the heattreatment, while the graphene platelets in the conventional GO paper andthe graphite flakes in the FG foil are essentially dead molecules. TheGO-derived unitary graphene entity or graphene single crystal is a classof material by itself.

The tensile strength values of three unitary graphene matrix compositeswith the final re-graphitization temperature of 1,500° C. are plotted asa function of the filler weight fraction for three carbon/graphitefiller types: CNT, expanded graphite flakes, and carbon black particles(FIG. 8( b)). Although adding CNTs to the unitary graphene matrixdecreases the thermal conductivity (FIG. 4( d)), the strength of theresulting composites increases monotonically with (actually proportionalto) the CNT weight fraction, reaching a value of 200 MPa that is oneorder of magnitude higher than the typical strength of flexiblegraphite-type materials. This is completely unexpected.

This suggests that GO molecules have a strong adhering power capable ofbonding to CNTs, creating a strong interfacial bond to assist in theload transfer and enabling CNTs to carry a significant proportion of themechanical force imposed upon the composite. It may be noted that epoxymatrix composites containing multi-walled carbon nanotubes as thereinforcement phase have never exhibit a tensile strength higher than 80MPa. This is partially due to the difficulty of dispersing CNTs in apolymer, to the extent that it has been extremely difficult towell-disperse more than 5% by weight of CNTs in epoxy. Beyond 5% byweight, CNTs could not be homogeneously dispersed in epoxy and thetensile strength actually begins to decrease with increasing CNT weightpercentage. The observation that CNTs can be well dispersed in thegraphene matrix up to 30% by weight is shocking, indicating outstandingchemical compatibility between GO molecules and discrete CNT filaments.Further shocking is the 200 MPa tensile strength exhibited by thegraphene matrix-CNT composite, a value that no reinforced epoxycomposite has been able to achieve unless the reinforcement phase (suchas high-strength carbon fibers) is well aligned in the loading direction(e.g. in a unidirectional fiber composite).

Examples 8 The Surface Scratch Resistance in Terms of Scratch Visibilityand Scratch Depth, and Hardness of Various Unitary Graphene MatrixComposites

The scratch test was conducted using the so-called Ford Lab Test Method(FLTM) BN108-13. This apparatus consists of a movable platform connectedto five beams with 250 mm in length. A scratch pin is attached to oneend of each beam. A highly polished hardened steel ball (1.0±0.1 nundiameter) is placed on the tip of each pin. Each pin is loaded with aweight that exerts a force of 7 N, 6 N, 3 N, 2 N, and 0.6 N,respectively. Driven by compressed air, the beams draw the pins acrossthe specimen surface and generate scratches. The scratch is made at asliding velocity of approximately 100 mm/s. All tests were performed atroom temperature. Although the test method requires that grainedsurfaces be evaluated, only the smooth surfaces of the specimens weretested in this study.

After the specimen plaques were scratched, they were evaluated with areflected light polarizing microscope incorporating a Xenon lightsource. An image analyzer with Image Analysis Software was used tomeasure the “gray scale mass,” which is the total gray scale value ofthe object. The camera objective lens is positioned at an angle of 90°from the scratch. The objective lens then registers a portion of thescratch about 1 mm long. The electron signal for each scratch line isthen integrated and recorded. The optical mass of an object, M, is thesum of the gray level values, GL, of all pixels in the object. Theindividual gray level values are assigned by the image analysis programin unit steps in the range of 0-255, where 0=black and 255=white. Theoptical mass, M, can be computed from: M=ΣGL_(i) (sum over i to n),where n is the number of pixels. The brightness of the object, B, isB=M/A, where A represents the area of the object. The percentage changein the brightness between the scratch and the background is the scratchvisibility, ΔB, given byΔB=[(B_(scratch)−B_(background))/(B_(background))]×100%. The depth ofthe scratch was measured using an interferometer. The magnification wasset at 5×. Depth measurements were made from the depth histogram of thescanned area. The scratches were also examined using a scanning electronmicroscope (SEM).

Indentation hardness tests were also performed on selected specimens.For the Rockwell Hardness test, the ASTM D 785 test procedure wasfollowed. The indenter was a round steel ball with 12.5 mm in diameter(Rockwell R scale). The Rockwell hardness number is a measure of thenon-recoverable indentation after a heavy load of 588 N for a period of15 s, and subsequently reduced to a minor load of 98 N for anotherduration of 15 s. Normal hardness is then defined as the load divided bythe projected area.

FIGS. 8( c) and 8(d) show the Rockwell hardness and scratch depth data,respectively, of several graphene matrix composites plotted as afunction of the filler weight percentage (FIG. 8( c)) andre-graphitization temperature (FIG. 8( d)). The Rockwell hardness datain FIG. 8( c) are found to be well correlated with the tensile strengthdata of FIG. 8( b). Again, the presence of CNTs can significantlyincrease the hardness of the unitary graphene matrix. The scratchresistance of the unitary graphene matrix can also be significantlyimproved by adding some CNT (20% by weight as in FIG. 8( d)). Thisimprovement is diminished as the final re-graphitization temperatureexceeds 1,000 C wherein the unitary graphene matrix alone is already ofhigh strength and hardness.

Examples 9 Thermal and Electrical Properties of Various Unitary GrapheneMatrix Composites

The thermal and electric conductivities of unitary graphene matrixcomposites containing various carbon or graphite fillers in differentforms are summarized in Table 1 below. Given the same final heattreatment temperature, all the graphene matrix composites exhibit betterelectric and thermal conductivities as compared to the baseline flexiblegraphite foil and GO paper.

TABLE 1 In-plane thermal and electric conductivities Re-graphi- ThermalElectric tization tem- conduc- conduc- perature Filler type, form, andtivity tivity Sample No. (° C.) wt. % (W/mK) (S/cm) 31-G 1,500 None1,610 4,200 31-G-AB 1,500 Acetylene black 946 3,550 particles,dispersed, 35% 31-G-MCMB 1,500 Particles, dispersed, 1,156 3,605 25%31-G-Coke 1,500 Needle coke, dis- 1,028 3,002 persed, 25% 32-G 2,500None 1,736 10,300 32-G-CNF 2,500 CNF, mat, 10% 1,550 9,213 32-G-CF-Uni2,500 Continuous carbon 1,250 7,250 fibers, unidirec- tional, 55%32-G-CF-W 2,500 Continuous carbon 1,143 6,037 fibers, woven fabric, 54%32-G-CF-Ch 2,500 Chopped carbon fiber, 1,057 5,454 mat, 45% 32-G-AC2,500 Activated carbon, 1,611 9,763 dispersed, 15% FG foil 2,500 — 5602,300 GO paper 2,500 — 920 3,500

As indicated in FIGS. 7( a) and 7(b), the presently invented unitarygraphene matrix composites do not have to go through anultra-high-temperature graphitization treatment to achieve a highthermal conductivity (e.g. K already=988 W/mK with T=800° C. and K=1,487W/mK with T=1,250° C.). Graphitization of a carbonized resin (e.g.polyimide) or other carbon materials requires a temperature typicallyhigher than 2,000° C., most typically higher than 2,500° C. Thegraphitization temperature is most typically in the range of2,800-3,200° C. in order for carbonized materials or pyrolytic graphiteto achieve a thermal conductivity of 1,600-1,700 W/mK. In contrast, thetypical heat treatment temperature (re-graphitization treatment) of thepresently invented GO-coated laminates is significantly lower than2,500° C. and more typically lower than 1,500° (can be as lower than500° C.).

For instance, polyimide (PI), if carbonized and graphitized for 5 hours(including 4 hours for carbonization at 1,000-1,500° C. and 1 hour forgraphitization at 2,000° C.), exhibits a thermal conductivity of 820W/mK. In contrast, we were able to reach a thermal conductivity of 988W/mK with a heat treatment of graphene matrix composite at 800° C. for atotal of two hours. This is very surprising and no one has ever thoughtthat such a low graphitization temperature was possible. Further, a heattreatment of the GO gel-derived unitary graphene-matrix composite at thesame 2,000° C. for 1 hour imparts a thermal conductivity of 1,680 W/mK(vs. 820 W/mK of the carbonized PI). Clearly, this is a dramaticallyfaster, less energy-intensive, and more cost-effective process. Theresulting products are also far superior to pyrolytic graphite. Theunitary graphene matrix composites, the unitary graphene layer itself(from GO gel), and the pyrolytic graphite are three fundamentallydifferent and patently distinct classes of materials in terms ofchemical composition, morphology, structure, process of production, andvarious properties.

In conclusion, we have successfully developed an absolutely new, novel,unexpected, and patently distinct class of highly conducting material:graphene oxide gel-derived unitary graphene material and unitarygraphene matrix composite. The chemical composition, structure (crystalperfection, grain size, defect population, etc), crystal orientation,morphology, process of production, and properties of this new class ofmaterials are fundamentally different and patently distinct fromflexible graphite foil, polymer-derived pyrolytic graphite, CVD-derivedPG (including HOPG), and catalytic CVD graphene thin film. The thermalconductivity, electrical conductivity, scratch resistance, surfacehardness, and tensile strength exhibited by the presently inventedmaterials are much higher than what prior art flexible graphite sheets,paper of discrete graphene/GO/RGO platelets, or other graphitic filmscould possibly achieve. These GO-derived unitary graphene materials havethe best combination of excellent electrical conductivity, thermalconductivity, mechanical strength, surface scratch resistance, hardness,and no tendency to flake off.

We claim:
 1. A process for producing a unitary graphene material, saidprocess comprising: (a) preparing a graphene oxide gel having grapheneoxide molecules dissolved in a fluid medium wherein said graphene oxidemolecules contain an oxygen content higher than 20% by weight; (b)dispensing and depositing a layer of said graphene oxide gel onto asurface of a supporting substrate to form a deposited graphene oxide gelthereon, wherein said dispensing and depositing procedure includesshear-induced thinning of said graphene oxide gel; (c) partially orcompletely removing said fluid medium from the deposited graphene oxidegel to form a graphene oxide mass having an inter-plane spacing d₀₀₂ of0.4 nm to 1.2 nm as determined by X-ray diffraction and an oxygencontent no less than 20% by weight; and (d) heat treating the grapheneoxide mass to form said unitary graphene material at a heat treatmenttemperature higher than 100° C. to an extent that an inter-plane spacingd₀₀₂ is decreased to a value of from 0.3354 nm to 0.4 nm and the oxygencontent is decreased to less than 5% by weight.
 2. The process of claim1, wherein step (c) includes forming a graphene oxide layer having aninter-plane spacing d₀₀₂ of 0.4 nm to 0.7 nm and an oxygen content noless than 20% by weight; and step (d) includes heat-treating thegraphene oxide layer to an extent that an inter-plane spacing d₀₀₂ isdecreased to a value of from 0.3354 nm to 0.36 nm and the oxygen contentis decreased to less than 2% by weight.
 3. The process of claim 1,wherein said graphene oxide gel has a viscosity greater than 2,000centipoise when measured at 20° C. prior to said shear-induced thinningand the viscosity is reduced to less than 2,000 centipoise during orafter shear-induced thinning.
 4. The process of claim 1, wherein saidgraphene oxide gel has a viscosity from 500 centipoise to 500,000centipoise when measured at 20° C. prior to said shear-induced thinning5. The process of claim 1, wherein said graphene oxide gel has aviscosity no less than 5,000 centipoise when measured at 20° C. prior tosaid shear-induced thinning and the viscosity is reduced to less than2,000 centipoise during or after shear-induced thinning.
 6. The processof claim 1, wherein said graphene oxide gel has a viscosity thatdecreases by at least 10 times when a shear rate is increased at 20° C.7. The process of claim 1, wherein shear-induced thinning is conductedvia a procedure selected from coating, casting, injection molding,compression molding, resin-transfer molding, extrusion, pultrusion,filament winding.
 8. The process of claim 1, wherein said step (d)includes heat treating said graphene oxide layer under a compressivestress.
 9. The process of claim 1, wherein said unitary graphenematerial has a thickness greater than 100 nm.
 10. The process of claim1, wherein said unitary graphene material has a thickness greater than100 nm but less than 10 μm.
 11. The process of claim 1, wherein saidunitary graphene material has a thickness greater than 10 μm.
 12. Theprocess of claim 1, wherein said unitary graphene material has athickness greater than 100 μm.
 13. The process of claim 1, wherein saidgraphene oxide gel is prepared by immersing a graphitic material in apowder or fibrous form in an oxidizing liquid to form an initiallyoptically opaque and dark suspension in a reaction vessel at a reactiontemperature for a length of time sufficient to obtain a graphene oxidegel that is a homogeneous solution and also optically transparent,translucent, or brown-colored, wherein said graphene oxide gel iscomposed of graphene oxide molecules dissolved in an acidic mediumhaving a pH value of no higher than 5 and said graphene oxide moleculeshave an oxygen content no less than 20% by weight.
 14. The process ofclaim 1, wherein said graphene oxide gel is prepared by immersing agraphitic material in an oxidizing agent to form an initially opticallyopaque and dark suspension and allowing an oxidizing reaction to proceeduntil a homogeneous and optically transparent, translucent, orbrown-color solution is formed, and wherein said graphitic material isselected from natural graphite, artificial graphite, meso-phase carbon,meso-phase pitch, meso-carbon micro-bead, soft carbon, hard carbon,coke, carbon fiber, carbon nano-fiber, carbon nano-tube, or acombination thereof.
 15. The process of claim 1, wherein said steps (b)and (c) include feeding a sheet of a solid substrate material from aroller to a deposition zone, depositing a layer of graphene oxide gelonto a surface of said sheet of solid substrate material to form agraphene oxide gel layer thereon, drying said graphene oxide gel to forma dried graphene oxide layer deposited on said substrate surface, andcollecting graphene oxide layer-deposited substrate sheet on a collectorroller.
 16. The process of claim 1, wherein the unitary graphenematerial further contains a discrete solid carbon, graphite, or graphenefiller phase dispersed in said unitary graphene material to form aunitary graphene matrix composite structure and said filler phase isselected from a carbon or graphite fiber, carbon or graphite nano-fiber,carbon nano-tube, carbon nano-rod, meso-phase carbon particle,meso-carbon micro-bead, expanded graphite flake with a thickness greaterthan 100 nm, single-layer graphene sheet, multi-layer graphene plateletwith a thickness less than 100 nm, exfoliated graphite or graphite worm,coke particle, needle coke, carbon black or acetylene black particle,activated carbon particle, or a combination thereof; wherein saidcarbon, graphite, or graphene filler phase occupies a weight fraction of0.01% to 99% based on the total composite structure weight.
 17. Theprocess of claim 1, wherein said heat treatment temperature contains atemperature in a thermal reduction regime of 100° C.-500° C. and theunitary graphene material has an oxygen content less than 5%, aninter-graphene spacing less than 0.4 nm, and/or a thermal conductivityof at least 100 W/mK.
 18. The process of claim 1, wherein said heattreatment temperature contains a temperature in the range of 500°C.-1,250° C. and the unitary graphene material has an oxygen contentless than 1%, an inter-graphene spacing less than 0.345 nm, a thermalconductivity of at least 1,300 W/mK, and/or an electrical conductivityno less than 3,000 S/cm.
 19. The process of claim 1, wherein said heattreatment temperature contains a temperature in the range of 1,250°C.-2,000° C. and the unitary graphene material has an oxygen contentless than 0.01%, an inter-graphene spacing less than 0.337 nm, a thermalconductivity of at least 1,500 W/mK, and/or an electrical conductivityno less than 5,000 S/cm.
 20. The process of claim 1, wherein said heattreatment temperature contains a temperature greater than 2,000° C. andthe unitary graphene material has an oxygen content no greater than0.001%, an inter-graphene spacing less than 0.336 nm, a mosaic spreadvalue no greater than 0.7, a thermal conductivity of at least 1,700W/mK, and/or an electrical conductivity no less than 10,000 S/cm. 21.The process of claim 1, wherein said heat treatment temperature containsa temperature no less than 2,500° C. and the unitary graphene materialhas an inter-graphene spacing less than 0.336 nm, a mosaic spread valueno greater than 0.4, a thermal conductivity greater than 1,700 W/mK,and/or an electrical conductivity greater than 10,000 S/cm.
 22. Theprocess of claim 1, wherein the unitary graphene material exhibits aninter-graphene spacing less than 0.337 nm and a mosaic spread value lessthan 1.0.
 23. The process of claim 1, wherein the unitary graphenematerial exhibits a degree of graphitization no less than 40% and/or amosaic spread value less than 0.7.
 24. The process of claim 1, whereinthe unitary graphene material exhibits a degree of graphitization noless than 80% and/or a mosaic spread value no greater than 0.4.
 25. Theprocess of claim 1, wherein said unitary graphene material containschemically bonded graphene molecules or chemically merged grapheneplanes that are parallel to one another.
 26. The process of claim 1,wherein said unitary graphene material contains no complete grainboundary therein, is a graphene single crystal, or a poly-crystalgraphene structure with graphene molecules being oriented along apreferred direction.
 27. The process of claim 1, wherein said grapheneoxide gel is obtained from a graphitic material having a maximumoriginal graphite grain size and said unitary graphene material is asingle crystal or a poly-crystal graphene structure having a grain sizelarger than said maximum original grain size.
 28. The process of claim1, wherein said graphene oxide gel is obtained from a graphitic materialhaving multiple graphite crystallites exhibiting no preferredcrystalline orientation as determined by an X-ray diffraction orelectron diffraction method and wherein said unitary graphene materialis a single crystal or a poly-crystal graphene structure having apreferred crystalline orientation as determined by said X-raydiffraction or electron diffraction method.
 29. The process of claim 16,wherein said carbon, graphite, or graphene filler phase is chemicallybonded by said unitary graphene matrix.
 30. The process of claim 1,wherein said unitary graphene material contains a combination of sp² andsp³ electronic configurations.
 31. The process of claim 16, wherein saidunitary graphene matrix composite has a physical density of at least 1.8g/cm³ or a porosity level lower than 5%.
 32. The process of claim 1,wherein said graphene oxide gel is obtained by immersing a graphiticmaterial in a powder or fibrous form in an oxidizing liquid medium in areaction vessel at a reaction temperature for a length of timesufficient to obtain a homogeneous solution composed of graphene oxidemolecules dissolved in the liquid medium, wherein said homogeneoussolution is optically transparent, translucent, or brown colored andsaid graphene oxide molecules have an oxygen content no less than 20% byweight and a molecular weight less than 43,000 g/mole while in a gelstate.
 33. The process of claim 32, wherein said graphene oxidemolecules have a molecular weight less than 4,000 g/mole while in a gelstate.
 34. The process of claim 32, wherein said graphene oxidemolecules have a molecular weight between 200 g/mole and 4,000 g/molewhile in a gel state.
 35. The process of claim 1, wherein said step ofheat-treating induces chemical linking, merging, or chemical bonding ofgraphene oxide molecules, and/or re-graphitization or re-organization ofa graphitic structure.
 36. The process of claim 1, wherein said unitarygraphene material has an electrical conductivity greater than 3,000S/cm, a thermal conductivity greater than 600 W/mK, a physical densitygreater than 1.8 g/cm3, and/or a tensile strength greater than 40 MPa.37. The process of claim 1, wherein said unitary graphene material hasan electrical conductivity greater than 5,000 S/cm, a thermalconductivity greater than 1,000 W/mK, a physical density greater than1.9 g/cm3, and/or a tensile strength greater than 60 MPa.
 38. Theprocess of claim 1, wherein said unitary graphene material has anelectrical conductivity greater than 15,000 S/cm, a thermal conductivitygreater than 1,500 W/mK, a physical density greater than 2.0 g/cm³,and/or a tensile strength greater than 80 MPa.
 39. A process forproducing a unitary graphene matrix composite, said process comprising:(a) preparing a graphene oxide gel having graphene oxide moleculesdissolved in a fluid medium to form a homogeneous solution; (b) mixing acarbon or graphite filler phase in the graphene oxide gel to form aslurry; (c) dispensing the slurry into a cavity of a molding tool orforming the slurry into a desired shape under the influence of a shearstress to create shear-induced thinning and molecular orientation; (d)partially or completely removing the fluid medium from the slurry toform a composite precursor; and (e) heat-treating the compositeprecursor to form the unitary graphene composite at a temperature higherthan 100° C.
 40. A process for producing a unitary graphene matrixcomposite, said process comprising: (a) preparing a graphene oxide gelhaving graphene oxide molecules dissolved in a fluid medium to form ahomogeneous solution; (b) combining a carbon or graphite filler phasewith said graphene oxide gel to form an impregnated filler shape underthe influence of a shear stress that induces viscosity thinning; (c)partially or completely removing the fluid medium from the impregnatedfiller shape to form a composite precursor; and (d) heat-treating thecomposite precursor to form the unitary graphene composite at atemperature higher than 100° C.